that's a bit like asking- I'm going shopping, how much am I going to spend??
What sort of boat? are you talking about alternator charging or 240 battery charger?? What size batteries are installed (ampere hours)? Fridge? Plotter? Laptop? autopilot? etc
Battery charging current can be what ever you want. If you have permanent 240v supply then you can use a trickle charger even a wall wart type transformer rectifier which will give 1/10 amp contnually or you can go for a massive charger capable of giving 30 amps which could at a guess recharge your battery in a few hours. (or any size between)
The electric panel connection for charger needs to be made for this current.
If you are going to leave the charger on while you are away from the boat you need to make a connection to the battery that remains when the master switch is turned off. This usually means a fuse in line near the battery and no connection to the main panel. Likewise if you go for solar or wind charging. Engine generator charging is done via the engine electrical circuits and only when engine power is on. We need more info....... olewill
