what exactly would turn the pump on and off?

The wiring would be CB in boat house to a switch and then to the pump. The switch would turn the pump on and off.

just size the pump appropriatly (1/2hp comes to mind) otherwise make sure to have an overflow of some sort, so if a hose kinks the pump isn't deadheaded....

You might find you have to water at full flow otherwise the pump cycles on and off ,, perhaps rather quickly, without a pressure tank. Pressure tank helps force the seal on the check valve too.

Curious - How far from the inlet to the pump & height ?

Foot valve will be about 15' from the pump, about 6' lower in elevation (4' under water).

The pump is a brand new Myers 2hp lawn sprinkler pump I bought for my farm and never used it. I know it is overkill for my purpose and I might be better off selling it and installing a small pump / pressure tank / regulator set-up like you see at HD.

That 2hp probably will give you about 50 to 60 Gallons per Minute ! Around here the typical pressure switch on that type of lawn pump would turn it off at about 40 lbs then back on at 20.
Without a tank, from what you've detailed.. thats way too much water. Its gonna kill itself cycling on/off rapidly without a good sized larger tank.