Moral to this is leave the fuel system well alone over water.
Leave the fuel selector alone unless you're prepared to lose that engine...any time. Not just over water.
I've witnessed far too many pilots who switch tanks just prior to takeoff...setting themselves up for a potential engine failure during or just after takeoff. Pilots who switch tanks approaching the airport when they really can't make a forced landing, due to nowhere to land. Who mindlessly swap tanks or move the selector to alter feed enroute or during an approach, without ever so much as putting themselves in a position to conclude the flight without the engine in question.
One should never move the fuel selector unless one is prepared to deal without that engine. One should never simulate a failure unless prepared in the same way.
Though I have a seaplane rating I've never thought it would be that much use when ditching a land plane.
Actually, a seaplane rating and the training that comes with it is excellent preparation for ditching an airplane. It is, after all, live practice at setting an airplane down on water. Think about it.