Hell Diver Trophy in Astro's Playroom
Dived into the water from the diving board.
How to unlock Hell DiverThis is done in Springy Spa - Cooling Springs (this is actually at the end of Bot Beach but it's faster to start from Springy Spa and go behind). Jump at the top of the highest diving board and then jump in the water.
If the gif doesn't work, here is a video: LINK