I liked this puzzle game a lot. Nice implementation of the classic riddle. Music was very fitting and relaxing.
It would be nice that after each level hints would be added on screen of what you learned from the previous levels. Start with no info, then after the first level is complete have a reminder that mouse eats grain, after the second level fox eats mouse would be added and so on. That way you can look at it and not have to memorize it all. Specially for a game jam having to memorize things is hard if you are testing multiple games in a row.
Other than that, I kept accidentally clicking on the sail for the trip back when I wanted to unload what I was carrying because the boat flips when it arrives to shore. Maybe making a boat design that has the sail in the middle so that it doesn't change places would be better. It's just a small detail though.
Either way great job!