This is a really awesome idea - I love the concept and the approach!
However, I did run into some pretty major bugs and UX issues.
Some suggestions:
1) Game controls: Use the mouse wheel to change items (weapon/repair tool)
2) Use the RMB to zoom in. It feels more natural and easier to control than scrolling your mouse wheel.
3) The UI hints dont seem to always pop up (telling you how to steer, accelerate and so on) - so my first play through involved me not knowing how to get out the harpoon.
4) My boat's life is....strange. On my first playthrough (after figuring out the harpoon) I lasted for ages before suddenly capsizing, but the second playthough my boat died after one collision.
5) Hitting rocks doesn't seem to damage the boat (that I can tell)
6) Maybe put a life bar for your boat on the HUD (or maybe even have it on your steering controls next to the radar?)
7) The dock at the beginning is cool but seems pointless. Maybe put some World Info there? Like a poster saying "Giant Fish wanted! £1000 reward!"
And have the core controls there?
8) The music had some issues, like there were two songs playing at once. Its nice, but might need a little bit of work.
Over all, like I said, awesome game concept that could do with a few tweaks! :)