Hydrocache is distinct from an underwater cache.
Hydros do not need to be under water but merely are on water/require a vessel to reach.
An example of a hydrocache would be an ammo box concealed on a small island in the middle of a river or lake. A cache-hunter would merely use a kayak, canoe, or jet-ski (or other type of watercraft) to reach the cache.
As always, make sure the cache is sealed when you're through to avoid moisture from getting inside.