Holy fucking shit. Swimming is not easy. I went today for the first time in years to start my triathlon training (I do the other sports almost every day). I tried to join a master's swim club today, and I got back a snooty email from their membership guy telling not to come out (and wanting to know if I could do a flip turn despite the fact that his website explicitly states that you do not have to know how to do a flip turn). At first I was indignant and pissed off, but now I have to admit that Marco was right.
I swam 100 meters in like 4 and a half minutes, and then I almost died. I wasn't wearing a heart rate monitor, but I would estimate that my heart was beating at roughly 300 beats per minute. I swam a grand total of about 300 meters.