well pump will not fill pressure tank
I looked up the specs for your pump and it's a high volume irrigation pump, not a pressure pump.  It's not designed to build pressure. And why even bother with a 20 gal pressure tank? It might be able to build enough pressure for the pressure switch to shut it off if you lower it enough.
Your last sentence in your OP is correct.   If you are using it with an irrigation system and a controller, many can support a &quot;master relay dry contact&quot; to turn on the pump.
Edit:  Just re-read your question. What are you using the pump for on a boat dock?  To hose down the deck, hull, etc? Fill tanks for domestic water use? Hose down the dock?   Given the pump you have, you may have to install a simple disconnect to shut off the pump completely when not needed.