Great advice from the guys as usual...
The ACR will (help) keep your house battery charged and the battery switch will keep you from getting stranded if your engine battery goes dead (and the house battery isn't depleted). I would do both the ACR and battery switch.
If you are partying and blasting the stereo at the beach, then running quickly back to the dock, the ACR won't do squat to keep the house battery charged. If you throw in a couple hours of skiing and have the engine running for a long time, the ACR may keep the house battery topped off.
Personally, I have several trolling motor batteries I can charge back up while trolling off the main engine, but it takes hours of trolling off the main engine to charge the trolling batteries. Even trolling for 1/2 -> 1 hour will throw enough charge back into the troll batteries to extend my day.
I also have an onboard charger, as mentioned above, to keep the batteries charged at the house. The worst thing you can do to a battery is leave it in a discharged state for an extended period of time, or leave it sitting all winter long with it slowly discharging. Best to keep it charged up every month or so.