Well I assume if they are not extinct, they at least know how to do it, don't they?
I was able to find this:
From a colleague who worked at National Geographic, and I'm not making this up:
"The sea-otter mating ritual is done this way. The male comes up behind the female in the water and bites at her nose to keep her still. The male then copulates with her. The couple takes huge breaths and then dives underwater quite deep, 100 feet or more.

And by the way it seems to be a quote from "Groundhog Day"