Bee Mario can fly a bit. When a special meter for this is empty, he cannot fly anymore until he touches the ground. Bee Mario can also stand on flowers and climb on honey. However, when touching water, Bee Mario will turn back to normal. You need this power-up to battle a boss in Super Mario Galaxy.
Bee Mario reappears in Ultra Mario World, however, this time the ability is not lost by touching water, rather the water gives him horrible swimming.