Water helmets are equipment created so that water breathers can come into Sandy's treedome or any place with no water. They first appear in "Tea at the Treedome," coinciding with the first appearance of their creator, Sandy Cheeks.
They are transparent, circle-shaped helmets filled with water, worn by sea creatures so they can be in Sandy's treedome. They also come in various shapes and sizes to comply with certain sea creatures.
Even though Mr. Krabs does not need to wear a water helmet to enter Sandy's treedome in "The Krabby Patty That Ate Bikini Bottom" since he is a saltwater animal, he is seen wearing one nevertheless while at her treedome in "Overbooked."
In "Pranks a Lot," when SpongeBob and Patrick are hiding in a sheet in Sandy's treedome, their water helmets are not seen in the said sheet.
During production of the episode, the animators realized halfway through that they actually forgot to put a water helmet on SpongeBob, so rather than redoing the whole episode, they decided to add a scene of him suddenly drying up and Patrick giving him a helmet.