For a game jam, I think the number of levels you had was just fine. You could easily expand upon this in the future if you wanted to.
I did get an interesting bug on one of the levels where I died and moved really fast and then both cubes appeared on top of each other. It was actually kinda cool! Maybe have a level or two where the cubes can interact in the same area and affect each other's progress?
I like the colored water idea, kinda shakes things up beyond just "two players controlled together". Would be cool to see other color-based obstacles and maybe a special move for our blue boi?