Ok this makes sense now. So a 15 amp Viper ESc can't actually handle 15amp continous indefinately without heating and possibly shutting down. So if you have a 4500mah battery back and running flat out with motor drawing 15amps, a 15Amp ESC will be safe for running this way for roughly 20min before the pack is exhausted. If you had a bigger capacity battery and ran it for longer at 15 Amps, you risk over heating. So If I got the 25amp ESC instead, I would still only be drawing 15amps so the ESC would be running cooler as its well within its design limits and hence I could run flat out for much longer than 20min ( assuming I had a bigger capacity battery) without risk of over heating?
I don't think scale boats will be running flat out for so long anyways.