The first section is kind of an introduction to this sort of water level. Avoid or kill the enemies, and move to the right. Climb the ladder at the end to move to the next section.
Move along the bridge until you meet a big cutter with an unlikely name. Tragically, when you consume him, all you get is a cutter, not a mega one. Still, use it to drop the elevator to the right and back track for a Sphere. Keep going to the door.
Grab the spear at the beginning of this section and follow the bottom path. You’ll reach a large gap, and on the right side you’ll see a switch. Go to the right side to hit it with the spear, and enter the revealed door for another Sphere.
In the next section you’ll be driving a boot. In accordance with standard video game rules, jumping off a bad guy leads to a bigger leap, enabling you to cross divides and such. Use the bad guys to get higher, reaching a secret star door before the regular door.
Ditch the boot and float up to a Sphere and a switch that will let you continue. Follow the path, avoiding the gulping eels, and look for some blocks above an eel that attacks from beneath. The last Sphere is nestled within. Avoid a few more eels, and you’re at the end.