I currently use this plugin.. very nice indeed.
There are some new products coming (summer 2014) out that aim to record HR data from waterproof wrist bands. I think Pulse-On (
http://www.pulseon.com) is one that springs to mind from memory
It might be worth thinking about how to integrate a separate HR data track with the data from Garmins (which as we know can't yet supply HR data underwater)
I have occasionally done this with a waterproof polar watch. For those one off sessions I can do separate analysis and there is no real loss. However if/when it bcomes very easy to get an extra HR track then it would be useful to integrate with the metrics you show in Pool Swimming. IMHO.