so Carlisle can swim across the english channel *spell* but i thought the fact that they have stone like body make them un-floatable?

Their skin is as hard as rock, yes.
But they still have the same mass as an average human.
And every average human can make themselves float in water.

...There's one crisis averted. :D

Swimming isn't the same as floating.

A 200lb person can swim and a 5oz rock can sink. The reason for that is that rocks can't swim.

We don't know how much ヴァンパイア weigh (their mass) but I do assume the are heavier than humans, considering how Bella couldn't even lift Edward's hand.
The equation of acceelration is acceleration=Force/Mass
As long as the body has the force (strength) to control their own mass (weight), them movement shouldn't be difficult at all, in any medium.