There are other things besides creatures that can harm you underwater. These include litter, oil, sharp rocks, whirlpools, currents, boats, and underwater volcanoes. Note; if you use Gillyweed, you only have 1 hour underwater. Then, if you are still underwater when it wears off, you should use the spell, 'Ascendio!' which will life you out of the water. Also, if you can Apparate, I don't know if Apparition is possible underwater. These are all things to be mindful of, so check to see if there are any of these things in the area you are exploring. Also, it's a very good idea if you have someone with you, or waiting at the surface.