I have actually witnessed people stealing a boat because keys were left in it a local restaurant on the water. of course, I didnt know it at the time, however 10 minutes later someone comes by screaming "their stealing my boat, their stealing my boat" as the thieves are off at WOT. Since I was there I provided the best description I could to the LEO's.
local marina when I was growing up, had 3 boats on trailers disappear from the lot one afternoon. someone drove up, hooked on to the trailers and drove away. in all 3 instances, the trailers had coupling locks.
For those reasons, I never leave the keys in the boat, and now in addition to a tow coupling lock, I now run a lock cable thru a set of wheels. the best you can do is slow a thief down.
back to a push button start. using a momentary switch for the start function along with a sustained switch for ignition and accessories is fine vs a keyed ignition switch. Its your boat, do what you want. however adding auto-start or one-button-start is complicated.
With IP65 ignition switches under $30 vs good IP65 rated push button switches and toggle switches at over $15 each, it doesnt make sense financially.