We found the hut and have a diving helmet, but need a full diving suit to get into the lake that has the key in it.
This is the low-spoiler guide for "Finding suitable underwater attire" puzzle in Dexter Stardust: Adventures in Outer Space. Unlike in normal walkthrough, you get gradual hints on how to solve the puzzle. Click the big button below to see your first hint. If the hint contains a spoiler, you will be warned about it before clicking.