In regards to charging batteries, I've seen people connect the battery charger leads to the posts on the battery switch. To me, this only serves if you have a single bank battery charger and mutiple batteries, as this would allow you to direct the charger to the battery of your choice through the battery switch. I have a multi-bank battery charger and connect the leads from the charger directly to the posts on each battery. This allows me to charge the batteries even if the battery switch is set to OFF.
As far as plugging a 30Amp connection into a 15amp Cricuit, that is fine. The boat has the capability of pulling 30a, however it will only pull as much as it needs. If the boat draws too much, the 15a circuit breaker in the house panel will flip first. I plug mine in to a 15a circuit in the boat yard all the time. Just consider what you're trying to run. Anything with a pump is going to have a high draw. Anything with a heating element will be even higher. So, try to avoid running an electric space heater, while cooking lunch on the electric stove with the water heater on, while you're running a couple of buffers and your wife is blow drying her hair.