I'm new to RC boats, but have raced cars and flown planes for years. I've never fitted a switch to isolate the battery.
On cars, the usual way is to have a small switch on the ESC, and keep the battery disconnected until it's time to race.
The small planes that I've built haven't used a switch at all - just unplug the battery. Saves weight, and one less thing to go wrong.
I've stuck with this for boats. Connect up the batteries at the lake, turn on Tx, switch on ESC.
I guess the difference is that, with cars and planes, it's normal to change the batteries several times during a session, whereas most boats run for long enough without swapping or recharging?
I wouldn't charge a battery inside a model either, incase something goes wrong. I've seen people doing it with SLA and a trickle charger, but anything else seems a bit risky. Batteries out before charging, and keep the battery/charger away from anything flammable. Maybe I'm just over-cautious?
Very valid points for planes and cars, and probably fast elecrics. But the normal run of scale boats tend to have the battery as a large part of the ballast arrangement, and not easily available. With a small boat, the placement of the battery can be very critical for trim, so having it in one place and keeping it there trumps everything else.
Most of my plastic conversions to date that have run on NiMH have a charger circuit built in. Usually a plug to match a cheap battery eliminator, a resistor to limit current, and a small bridge rectifier so I don't have any problems remembering the polarity of the eliminator. As I shift to lithium, this might well change.