I like most would recommend not swimming in a pool with a HR monitor. Here is the reason: I tried swimming with my HR monitor last night. I have the Polar s625x. This is the second time that I've tried swimming with my HR monitor. While I'd do it again I'd have to say that it consistently slips down to my waist when pushing off the walls doing flip turns. I pretty much have to gently tap off the wall to prevent it from slipping. In the future I plan to get a full suit to train in so the water doesn't catch the HR monitor.
Also, most swimmers don't rely strongly on HR as much as they do time. Timing the intervals of your sets is a better indication of how fast you are getting over time. However, I'm not discounting the use of monitoring HR. You can also try wrist or neck and count your rate if you don't want to hassle with a HR monitor. Just know that this method isn't as accurate.
I would train with it under my wetsuit when doing open water. Note that the HR isn't consistently measured through out the work out. I use it to look at my HR between sets to help gauge my effort level.