So Joe...you might have around 200ah's as a house battery bank. Just a small 15-20amp smart charger should suffice well since you are mostly at the dock.
I agree with dawg about adding a breaker. I would NOT go for the echo charge in your case as that is $150 bucks of expense that you just dont need. Instead...I would wire your two house batteries in parallel on Position #1 of your battery switch...then wire your starter into position #2.
Wire the charger into the battery switch and MOST of the time just leave it on #1 but give #2 a chance at a few amps now and then if you have not done a lot of motoring or if you get a low voltage reading (12.2V or less).
I think this will be the least complicated and least expensive setup for you and will work just fine for your particular case. BTW...the IOTA 15amp with smart charging would be a good unit to consider:
The DLS-15/IQ4 features an internal IQ Smart Charge Controller which automatically provides three-stage battery charging for safer charging and longer life for your system's battery.