I am speaking only for myself (personal opinion) - the idea of game jams is to try out ideas and get feedback. I've been to many game jams (started this year) and this is perhaps the first submission I see focused entirely on graphical realism.
The choice of making a large Windows build already limits the accessibility of the game => smaller amount of players who are going to give you feedback. By using high polygon assets you limit that pool of potential players further. Disclaimer: I could not play much because of lag which was most probably caused by running the game in a sandbox (in other words I make no comment on how optimised the game is).
So before we even get to the important part - your ideas, there is already a very small portion of players who can access the game to give you feedback.
Unity is a popular and proven technology. What matters is how you use it. What is the creative idea? You wrote that the main goal is to have the player sail to an island and then walk inside the island (movement from point A to B). What distinguishes your game from other pirate/sailing games?
I advise you to strive for graphical realism only after players have validated a stylised version of the game.
Good luck!