Great list! There are some things that I personally would like to talk about more...
-Waves
How would they affect your raft?
-Pirates
What would the pirates be like (Modern, old school, what would they do)?
How rare would a pirate attack be?
-Bow and arrows
This idea I do not personally like because fighting the sharks with the spear is such an original thing that it should be left like it is. (Other melee weapons would be cool)
-Multiplayer
Raft multiplayer would be a very interesting concept. In my opinion the multiplayer should be like this...
1-4 People spawn on the same raft
There are multiple sharks
Thats all I have to say...