Just completed the game. I like the idea of the ending with global warming and water pollution but once you have completed the game, there are no other goals for you to aim for. I think it would make more sense if there was a world map (scaled down so you can go around the world) and have hints to the story line (global warming caused sea levels to rise and submerged continents etc). For example "Australia is submerged" so if you were to travel to the location of Australia on the map, it will be an open ocean which will confuse new players at first as to why this is as it is and then they will find out when they find the radio tower at the end of the game. I also think that the islands should be more like the tips of islands. For example if you where to travel to Nepal/Mt Everest on the map, you might find the tip of Mt Everest, also hinting at the story line.
The shark, thirst and hunger currently doesn't add enough difficulty to the game, as the game progresses the shark should become stronger and possibly include several sharks at once into the game. Currently The shark does about a quarter damage of your health and you can cancel its attack by hitting it with the spear before it hits you. It also destroys your raft at only one block which is easily repaired. There simply isn't enough risk and loss in the game making it very easy. You could also make the game harder by having to maintain you raft and add some sort of float mechanics so that the construction and placement of objects actually matters. During storms you could have to constantly be repairing and fixing your raft from holes and other issues with the sails for example.
The game can be beaten very quickly as well, and there should definitely be more emphasis on exploration and dangers such as the shark. More items and materials to be found would also be a good improvement to make the game last longer. I also think an introduction of a pickaxe would be appropriate, for acquiring materials underwater rather than the hook. The hooks is for scrap, axe for trees and pickaxe for mining minerals and scrap underwater.