It's so nice to see a game made in Pico-8, we made ours in Picotron! It's a very very cool arcade game. The movement feels nice the art is very cute and the particles behind the floaty and the lines on the water to simulate it moving add a lot to the game (even if they are a small feature).
I got 955 after a couple of tries, however since there doesn't seem to be a max amount of objects thrown in the pool, you can just wait until the time is almost over and capitalize on the combo:
