First of all, great game. Later today, I'm going out to buy a PayPal card to donate $10 because this game has so much potential!
Now, onto the suggestions:
* I would like it that if you make a mini "pool" inside your raft, that the shark can't just phase through the blocks, maybe he would start destroying blocks trying to get to you. But, ghost sharks are too spoopy 4 me.
* Another addition I think would be if your raft wasn't connected to another part of your raft, they'd drift away, because when you just have two different rafts unconnected and they just stay stagnate kinda ruins the feel of "building a base at sea".
*My final idea is for a "mini-raft", or "boat" which would basically allow you to move around quicker in the water. The shark would come to attack it of course, but the mini-raft would act as a health buffer and would also allow you to escape from the attacking shark.
Thanks for reading my suggestion!