Drawing current off the centre of a 24v battery bank is a recipe for disaster, unless there is a means to replace this current.
Your R/H batteries will go flat, and the L/H batteries will be overcharged.
First thing to do is remove all the batteries from the boat.
Connect all four batteries in parallel to give 12v and then charge then up on a 12v charger.
This will even out the battery charge across all the batteries.
Then get the batteries tested.
You need to change the connection points for the 12v output to the posts on the L/H batteries, not the R/H batteries.
You then need to fit a battery charge equaliser.
The Redarc CE10 is the one I normally use. Redarc is in Australia, so don't know if they are available where you are.
10A Charge Equaliser | REDARC Electronics
The CE10 will keep the centre tap voltage at half the battery voltage. If the alternator voltage is 27.6v, the centre tap will be 13.8v.