Difficult stage, mostly due to the difficult boss fight. Movement through the stage isn't terribly difficult, but there is a good amount of luck required to pass through without damage. The boss fight can also waste a lot of time, like armadillo.
The first turtle can be a bit of an obstacle, there are a couple ways to go about getting past it. You can dash, kill the water-skimmer, jump to land in front of the turtle, then dash jump (without dpad input) and wait a handful of frames before pressing right and getting over the turtle. Another way is to, rather than land in front of the turtle, dash up to the turtle and jump when you get into the right position. Both are basically equivalent, just try both and see which one you can pull off easily.
Another way through the turtle which can potentially be faster is to kill it completely. Hold a charge upon entering the stage, use a mid-charge shot in front of a dash shot to get through the water-skimmer and hit the turtle, then shoot it a handful more times in a second dash forward, swap to ice and fire inside the turtle at the last moment. If done right, you can kill the turtle without taking a bonk. This is a lot of inputs with a fairly narrow margin of error (you only have to release the dpad for ~8 or so frames before you can get over it in a jump), so be careful before putting it into your runs that you aren't actually losing time to pull it off.
You can also use strictly buster shots to accomplish the same thing if your mashing is up to the task.
Landing near the left edge of the platform the second turtle is on lets you just dash jump right over with no fear of a bonk. Then, you can try what's called the Lobstrocity Leap. It's normal to simply dash off the platform the turtle is on, and do another dash jump over the spike pit, but you can save that one dash input (1+ frames saved) by jumping from the edge of the platform the turtle is on. You can clear the spike pit cleanly this way.
The next section, with all the seahorses coming up, can be done without firing anything if your movement is clean. If you're worried about bonks, you can swap to tornado or c.sting or something and kill them as they rise up so you can get past them without getting hit, it doesn't cost anything to do.
For the first fish sub miniboss, as it rises up stand far away from it. Being far away enables it to summon 4 snake enemies to attack you, which it can do instead of trying to suck you in. When it rises up, fire a rolling shield from far away, then fire another rolling shield and dash toward it as it begins summoning the snakes. Swap to c.sting and you can just spam shots point blank. Be careful though, it should only take 6 actual hits with c.sting to kill it, so count the flashes which indicate a hit. Upon kill, each of the snake enemies will die as well (unless they were fired already) so they can all drop something. If a stray c.sting shot is fired, it will take a while to get off screen while contributing to lag and preventing you from swapping to buster. You want to swap to buster right after the kill because if ammo drops you want to collect it to reduce lag, but you don't want the game to pause to refill your c.sting ammo.
If the fish sub sucks you in right away, you can either spam c.sting directly and just take a hit, or fire a c.sting shot to the right, dash left, then fire right again, and repeat. It's difficult to do quickly, but octopus can be a dangerous fight, so having all the health you can hold on to is helpful.
There's another fish sub very shortly after the first, and it's a little easier to deal with. Just stand in front of it as it rises, and begin firing c.sting at it. It can do one of two things from this position, use its searchlight, or blow you away from it. The searchlight is very laggy, but you don't have to do anything about it, just stand still and shoot it until it dies. If it blows you away, let it blow you across the gap (you won't fall in if you don't touch the dpad) and once you're safely on the other side, just walk to the right while shooting. You may slip off and fall into the pit, but as long as you press jump you'll kick off one of the walls and get out easily. To avoid falling in, just release the dpad once in a while, very briefly.
The next thing you encounter in the stage is the cruiser dropping bombs. You don't need to really do anything with that thing in any%, just dash by and try not to get hit. Taking one hit is fairly standard, and shouldn't be a big deal.
Next up is the utuboros, it will begin burrowing toward the surface when you get near the top of the small hill in the arena, so get there asap. You can save 5-10 frames by getting the utuboros off screen afterwards, by either dash jumping to the right and off the right pillar, circling back to the left, or by jumping on top of the left pillar and standing for a few seconds before dropping down.
Stand at the bottom of the hill facing right, and when the utuboros reaches the surface, you'll see its spiked head appear through the ground, fire a tornado and dash jump. Don't just hold right though, you'll despawn the tornado, so pause briefly over the utuboros and continue moving again when it starts to die. Getting it off screen when it dies cuts out all the lag from it breaking apart against the ground.
Waterless route Edit
The fish sub minibosses can be done exactly like in any%, or optionally you can use charged c.sting before each of them. Just be careful with your ammo use, if you're using charged c.sting you shouldn't just spam shots, the subs have invincibility frames so space out your shots every 10 frames or so.
After the second fish sub (after the screen starts scrolling again, charging contributes to lag), start holding a charge and swap to tornado. Dash jump underneath the whale cruiser and fire the charged shot, it should die very quickly. A good portion of the time there will be a depth charge in your way, there's not much you can do about it except wait on the ledge to the left until it's clear. Once it dies, the cruiser will start sinking. It will destroy the spike pit and the ground below. To reduce the lag, you want to get all that stuff off screen until the final section of the ground is being destroyed, you want to see the final section break, but you don't want to stand on it. Drop down along with the cruiser and charge up another tornado for the utuboros. The charged tornado will kill the utuboros very quickly, then climb the wall to get the utu off screen to break apart, swap to boomerang and go grab the heart.
Charge up yet another tornado before reaching the next utuboros, and release it a little left of the top of the hill in the arena, it will kill the utu before it even gets a chance to surface, then just continue on to the right, charging up again for the Octopus fight.
Waterful route Edit
There's only one difference from the waterless route for this stage, and it's the fish sub minibosses. Since you don't have c.sting, the primary method of damaging them is gone, so you have to use something else. Rolling shield deals the same damage as c.sting, so on the first one you just stick with rolling shield the whole time. Charge up a spark shot for the second sub, fire it to deal damage and break its searchlight, then just continue using spark to finish it off. You don't want to use rolling shield again here to save your ammo for the Octopus fight, unless you picked up a lot of ammo from the first sub.
Everything else can be done like in Waterless.
Octopus fight Edit
Octopus always opens with a jump and torpedo shot, so dash jump toward him and fire a rolling shield. You want to open with a rolling shield rather than a dash shot because of the energy drain attack. You can jump into the energy drain and mash buttons to escape, taking a minimum of 1 damage. If you open with a dash shot, then even a single health drained will force you to have to hit octopus once more, so you want to open with the 3 damage rolling shield to have a one hp buffer on energy drains (in 100% open with a charged rolling shield for 4 damage, even better). In general, it's pretty common for octopus to use energy drain 0-1 times in a fight, so it's slower to cut off his arms. If you have difficulty getting out of the energy drain taking only 1 damage, using boomerangs to cut off Octo's arms becomes a valid tactic.
Octopus's main attacks are firing 2 waves of 3 torpedoes and firing 4 homing shots. There's an extended animation when he's going to fire the homing shots, so you can get very close before he fires. Touching his arms doesn't hurt you and the homing shots spawn pretty far from him, so getting close prevents them from hitting you initially. With practice, you get familiar with how you can be moving around the arena to avoid most of his attacks. Getting hit by torpedoes or the homing shots isn't so bad though, they only deal 2 damage, so if you're going to get hit by something, get hit by one of his attacks rather than bonking into octopus.