Also I have another question: is a beach a good setting for the jam?
I want to make a game where the protagonist visits a beach during the day AND the night.
During the night they might try go underwater.
Any setting works as long as it's underwater so yes, you could use a beach. But walking on a beach is generally being near water, not under. So unless you add some underwater elements [mermaids, deep ones, kelpies, whatever], let the chara dive a whole lot, have an important traumatic event that happens in water or anything else, really, that would make it a pivotal part of the game, it won't qualify.