best way to level up swimming?

[Uthgaard]

Go into third person mode, hold page up or whatever your chosen hotkey is until you move upward when you swim. You want to angle up enough to not sink/drown, not enough that you start to jump out of the water. Next turn on autorun, and hold a directional key so that your character spins, and then tab out. If you tab out while swimming upwards and spinning with a directional key (not mouse), your character will continue to swim in a circle.

Hope this helps.