Upon entering the water, your body melts and becomes the size of the water mass. If you try to leave the water you will shrink down to a medium.

Your movement turns to 75ft and you are invisible (if you want to be). Yes, you can control the water you are in, but the most you can do is turn boats and drown people (would be scary for a water campaign).

When you leave water your speed again drops to 10ft.

WaterForms are very rare, but traces of them can be found throughout history everywhere. They can be anywhere from small to large and can appear almost exactly like there original form, except more watery.

Something happened (in water) that made the body die, but not the spirit. afterward, with nothing to possess, you simply become water and are able to control it.