a battery charger IS an ac to dc converter, with the difference being, a charger is better controlled, and a power supply/converter delivers a constant uncontrolled output...
you have 30amp shore power available. thats good... bring it into your boat.
no matter if the AC switch or receptical in the boat is only 15amp, you wont burn up your 15amp switch just because you connected to 30amp service.
and you wont be running anything in your small boat that will pull more than 15amps thru the switch anyway.... except maybe an extra large heater...
so why dont you want to hook a "charger" up to your battery?.... it will allow you to keep the battery fully charged, AND at the same time, run any DC loads you have without depleteing the battery level... any converter/charger that is big enough to power a dc applience without a battery in line, is by default, plenty big enough to power the appliance AND charge the battery at the same time.... as it takes a more powerful "12v power supply" to power something stand alone, than it would take if you have a battery in the system....
a charger is the way to go.... you dont need an inverter, as you have ac available when at the dock, so you only need to convert the ac to dc....
you can use a small inverter when away from the dock, so you can invert the dc battery power to ac power to run a laptop, or other ac demand, but its an inefficient and short lived way to go with a small battery bank... other better options are available for this.