The first thing to remember before going underwater, is this. How are you going to breathe? There are several ways, and the most famous examples of them are in the Triwizard Tournament, during the 2nd task, where the champions had to go underwater to reclaim what had been taken from them. The first way is to use the Bubble-head charm/ This will create a bubble of air around your head. The second way is to use Gillyweed. Gillyweed is a magical herb, that when consumed, gives the eater gills, flippers, and webbed hands, which will allow you to move smoothly through the water. The last way, which was not used in the Triwizard Tournament, is to become an Animagus. This is extremely difficult, and takes a long time to learn how. Also, you will need to be able to transform into an underwater creature. If you are going to be in both fresh and salt water, I would recommend turning into a salmon. They can be in fresh and salt water, unlike most marine creatures. If you are only doing one or the other, turn into either a Sea Otter, or a River Otter. Note; the animal into which you transform is permanent. You won't be able to become any other creature.