Thanks for the proofread.Just so folks aren't confused, @Hayden Watson's post contains a simple typo. The switch is rated for 20A not 2A. 2A is wholly insufficient for 1500 watts at 120v.
I fixed in to eliminate the confusion. 
De-powering water heater for winter
- Thread starter D. Inscho
- Start date
I fixed in to eliminate the confusion.
If you install a breaker that can be switched off, it will be best for the breaker if you are not connected to shorepower when you turn off the switch. If the element is heating when you open the breaker, it will have a current close to its rated current flowing through it that will cause arcing with will build up carbon deposits on the contacts which will cause the breaker to fail. The switch can also arc but their design is built to handle it better and they are far cheeper to replace.