As much as it's fun to be Britishly superior to us Americans, I'm pretty sure it doesn't apply in this case.
Unlike rabbits and hares, there is no scientific distinction between tortoises and turtles, so the British definitions are no more valid than the American ones.
Oops, not cetaeans. What do I mean...it does begin with C...okay I looked it up and I think the word I was after was "chelonian" or something, lol.
Tortoises have legs and live on land, turtles have flippers and live in the sea (not counting the bit where they come on land to lay eggs as that's just one quick trip ;)). I'd say that was enough of a difference that you want a different word for them, otherwise how would you know what someone was talking about if they didn't clarify? ;)