game of thrones - Can White Walkers swim?

In Game Of Thrones Season 5 Episode 8 (Hardhome), we saw :

Jon Snow and others escape from white walkers using boat.

I wonder why :

the White Walkers couldn't enter into water or swim? Can't they just turn water into ice with their touch?

Was it intentional :

to let Jon Snow run away after they saw his sword (Valyrian steel) could kill White Walkers?