You need a 2 bank charger made for two batteries. Ignore the selector switch and wire the charger to the batteries directly.
Each battery gets its own charge line and its own positive and ground hooked right to the battery.
It's good to have a selector, but not for charging.
How much of a charger depends on your boat use, if you have a bass boat and troll for hours and hours then sleep for 6 hours, then back out at 4:30 am for a tournament..then you need a big expensive one!
but if you use the boat for a day then might not go out for another week, then not as big. Since you only have two batteries, I'll assume this isn't a bass boat.
So you just need a simple 2 bank charger that's fully automatic.
I'd still get a decent one though so you can troll all day and it'll be fully topped the next morning.
https://www.amazon.com/ProMariner-43020-ProSport-Generation-Battery/dp/B00F5EBS66?ref_=fsclp_pl_dp_3&th=1The one up there ^ would probably do you all you need, but if you want a cadallac charger there's this, 36amps so you'll have your battery charged up quickly, enough for any tournament.
https://www.amazon.com/ProMariner-43020-ProSport-Generation-Battery/dp/B00T59O20G?ref_=fsclp_pl_dp_3&th=1
