I don't know relays, so I can't give advice to that but from classical electronics
I=P/V
where I is your current P is your Power and V is your voltage.
edit:
With that said, if this were a fuse for your fuse box you typically only want the fuse to be at 80% capacity, I can't imagine relays being much different. So in this case a 15A fuse would only be at about 75% capacity for your water heater. Again I'm not an export, and I would get confirmation for that.