Same for me when I'm swimming. I pay attention to what the water is doing around me, feel the water I've gotten a grip on, concentrate on sliding my hand into the water as smoothly as possible - and that's... it. I think the Zen folks call it meditation in motion. I get into a similar no-thoughts state on a motorcycle, but I'm paying more attention to what the vehicles around me are doing, what they might do, road surface, sight distance. Same state on a track (plus butterflies, in spite of it being a safer environment than mixing it up with oblivious car drivers).