And of course, what Mega Man game would be complete without an aquatic Robot Master?
As an aquatic stage, this is one of the few places in the game where you can actually get some good use out of the Rush Marine you got from killing Toad Man (you've beat him, haven't you?) It will ease the pain considerably.
The big threat you have to look out for in this stage is the Whale mini-bosses. You'll fight two of these guys, and you always seem to encounter them on unforgiving terrain laden with spikes. Jump to dodge their attacks, firing as you jump. The main secret to success here is stay on the attack, and don't let yourself end up on the spikes. They take a lot of damage before they go down, but if you stay on your toes they aren't too tough.
After the second whale fight, you'll find a hole in the ground. Fall down it, dodging the spikes as you do so. At the bottom, you'll get the Wire item, which can be useful in certain situations throughout the game. You'll have to do some backtracking through the level once you get it, but it's worth it for obtaining this somewhat useful piece of equipment.
Dive Man himself is a relentless fighter, but you've got TWO weapons he's weak against. Skull Barrier is what he's weakest against, touch him with the barrier activated but don't stick around or fall right on him. He's also weak against Dust Crusher if you prefer fighting from a distance. He'll take more damage from Skull Man's weapon, but it's (obviously) harder to it with. Dodge his torpedoes and his body when he jets across the arena, and let loose with either of these weapons.