I used to train with a club and swim for my high school, have some medals too.
Swimming is exhausting. Swimming for half an hour at a reasonable pace with proper form takes it out of ya. I used to do it on mornings for one hour, and I was washed out for the rest of the day.
I swam 2.5 kilometres in an hour, in an out door pool. My willy hurt from the cold, took about 2 hours to recover.
The chlorine fucked my skin, really dries it out and feels nasty.
Stick to the treadmill. At least you can watch the T and A walk past, chat, monitor heart rate, drink water, watch tv, listen to music, read magazine etc