Swimming is hard though. I spent 40 minutes doing laps in the pool and I've felt nauseous ever since I got out. I haven't really swam since I was a kid but it never seemed hard then. Then again, I never swam for the purpose of cardio and I'm in pretty lousy shape right now.
I did front/back/breast stroke as long as I could and when my arms started to be too tired I got a flutter board and just killed whatever energy was left in my legs.
Both my legs and arms are a gummy right now.
I have to say that it's much more enjoyable exerting yourself in the pool than running. It's nice not being all sticky and getting a headache from the heat.
Need to get my breathing right though. Inhaled water more than once tonight =/