You can get 12VDC relays and water-proof switches at most any automotive parts
make sure it can handle a high load amperage. I sized mine for 30-40A. Here is an Amazon item you can match to: https://www.amazon.com/gp/product/B00E358KEG
any 12VDC switch that is waterproof. This is a low amperage switch as it simply controls the relay. You can even keep the momentary switch in there if you like, but you will have to hold it in to keep the pump running!
Another Amazon example: https://www.amazon.com/Baomain-Toggl.../dp/B01IYD8SHQ
The waterproof part is just a rubber cover/condom that fits over a standard toggle switch.
How to replace:
To replace the relay just pull the old relay out of the socket and replace it with the relay you now have. You can push the old button in and hear the pump run to verify things are correct.
The relay is under the sink in the 2015 450F.
in the Starboard forward locker. You will find a round hatch
between the locker and the shower. Inside that round inspection hatch
is the back of the switch. Remove the existing switch and put in the new one. You may need to drill the hole larger. Be careful not to drop the wiring
as it may be difficult to retrieve.
Both relays are in the aft head
inside the cabinet under the sink. Look near the bottom of the opening.
The switches, and access to them, are obvious on this side.
Hope this helps!