Really- I don't warm up. I do some yoga-type stretching before the swim starts, but that's about it. If it's open water and an in-water start then I'll swim for about 3 min before the start. Otherwise there's no warm up for me. Even for the IM I only had a 3-4 min swim as a warm up. With transition closing and the wetsuit battle that ensues before it closes, there's not much time for warming up unless it's 40 min to an hour before the race (and then really, what's the point?). Shorter races I don't even bother. They're over fairly quickly, and I figure it's not worth tiring myself out to get a warm-up in. I always start the swim slow for the first couple minutes- so I figure that's my warm-up.