I've read this entire thread.
My thoughts are......you've been informed from previous posters you have a problem. Disconnects and on/off switches provide a work around for the problem.
Sorry, the dealer's comment is bogus....and "standard" for dealers who don't give a crap....you said you're not impressed.
I have 4 cars, a motorcycle, and a boat. I never hear "clicks" when I re-connect a battery. If I did, in addition to a discharge problem, I'd be looking for the reason. Also, one car and my truck can sit for weeks with no startup issues. It was the same with my boat for a few years. ( battery dis-connect installed now)
Something is on that shouldn't be.
In a car, and it's time consuming, you can pull individual fuses to try and isolate the circuit with the problem.
I don't know what your boat has....but if you can do that and the click goes away, that might be the circuit causing the issue....
food for thought.....I don't think most new boats come with battery disconnects, do they?....