"Can you swim?" Siggeir said as their heads appeared above water. "I'll meet you at the shore!" he shouted as he turned into a ball of gas, just narrowly escaping the waves.
Spoiler
Gaseous Form
I love being a vampire
Oh and I lose 1/3 of my hit points for being immersed in running water. 99 HP left. I regain 5 points each round.