The watch is water resistant to 5ATM. In practice this means it is suitable for swimming (not diving). http://en.wikipedia.
The buttons should not be pressed underwater but it is extremely rare to have any water ingress through the buttons. All swim watches are water 'resistant'. Really, only divers (SCUBA or Free Divers) use true water 'proof' watches where you can press all buttons deep in water.
Salt water pool:
Salt water can be very conductive it will not harm the watch itself but... with the Poolmate HR, it could short out the heart rate belt signal before it gets to the watch. We have found that wearing a neoprene swim vest or wetsuit over the strap will prevent this and the watch should work correctly then.