You do not need a battery switch just because the boat has a bunch of batteries. A 24 volt trolling motor needs two batteries connected in series like shown below: That does not require a switch of any kind.
Just because you replaced a battery and whatever it powers doesn't work is also no reason to suspect you have a battery switch. As Don said -- if there is nothing between the POSITIVE post of the battery and wherever a red cable disappears, you probably don't have.
I would be more inclined to be looking for a built-in (called an on-board) battery charger that has a regular AC power cord laying about. You plug that in at the dock to charge the troller batteries. The engine will take care fo the starting battery.