If they had biathlons where you ran and swam, I'd do those all the time. I don't think swimming is any more difficult than running...just different. The bike part is what never got me interested in triathlons...too much crap to buy and maintain. For a running and swimming biathlon all I need are my shoes and my speedo!
They do actually have this kind of a biathlon, but there is only one that I know of...it's in Glen Echo, Maryland. I think it's a 5k run preceded by a pool swim of some length.