I have a polar HRM with the soft strap that has a small transmitter. To tell you the truth, I often forget that I'm even wearing it I find it that comfortable.

I've used it biking and running, but don't feel the need to use it swimming, so I can't tell you what it performs like in the water. I love my HRM and get very annoyed when I forgot to take it or put it on. I actually have two HRM's, the older one being an earlier polar HRM monitor with a plastic chest strap that goes halfway across your chest. I found this heart rate monitor not as comfortable. Not only that but my older HRM has far less lovely functions than my current HRM.

The only downside to my HRM would be the size of the watch, but I only wear it whilst exercising, so its not that big a deal.