They are circle-shaped helmets filled with water, worn by water breathers/sea creatures so they can be in Sandy's treedome without dehydrating from the lack of water. They also come in various shapes and sizes to comply with certain sea creatures.
In their first appearance in "Tea at the Treedome," they contain two openings (with the difference between their usual appearance being the hole at the top) so that SpongeBob and Patrick can easily drink the tea that Sandy made for them.
Role in series
Sandy originally used water helmets for SpongeBob and Patrick when they shriveled up in "Tea at the Treedome." Later, they were used conventionally for any sea creature who came to her treedome.