This game does have some great bones. Its a different concept then most when it comes to survival games.
Adding to the difficulty either in mode or timing would be a nice addition.
There could always be new hostile mobs that show up/different types or numbers of sharks. Changes in your environment such as a storm/heatwave/waves could also add to the experience. During the environments changes it could be easier to collect certain items while harder for others. For example a heat wave could make it harder to grow tree's/plants. Maybe if its a storm and raining plants grow better but its harder to catch fish and the aggressive mobs are more annoyed/active.
These are all just suggestions, so far the game is rather enjoyable and I love the concept. Keep it up<3