Day 4 update:
I'm starting to worry that I might not be able to finish in time, so I'm going to cut back a bit on the idea. Instead of diving beneath the water, you'll be catching things that surface/jump out. I think this might actually make slightly more sense too, since you're trying to capture a whale.
Anyway, here's how the mechanics will work:
1. Fire the claw from the cannon. I'm thinking there'll be some control over power here, or something.
2. While in the air, you control the claw. Hitting seagulls will temporarily increase your bonus multiplier, and hitting small fish catches them. Large fish require you to zap them with your shrink ray until they're a manageable size. Naturally, this will devalue them a bit so bigger claws can get more from them. You'll also probably have to buy the shrink ray as well.
3. Your claw eventually falls into the water.
4. The money earned can then be spent on upgrades to the cannon and claw.
5. Once you get far enough out, Big Red shows up for a boss fight of some sort, where you must shrink her down while dodging a few attacks.