^I've been there. I took a header into a barely frozen over pond when I failed to negotiated a sharp corner after a downhill. I was braced for impact, but was then shocked to be swimming. Or at least until I stood up. It was only 1.5m or so deep. It took me a few seconds to scramble out. Luckily it was not a cold day, and I was less than 10km from the warm hut.

A note on the speed issue: As Juha noted, skate skiers can go pretty fast. Some races are pushing 30km/hr average. Pro MTB XCs are usually closer to half that (though usually on more technical terrain).