11.1V battery charging with optional solar panels
You need two things:-

An MPPT solar charger designed for 3 cell Lithium battery.

A BMS (Battery Management System) board with balancing function, wired onto the battery. This keeps the cell voltages the same so the whole battery can be charged at once, and protects against over-charge, over-discharge and over-current.


You should also have a voltage monitoring device to warn you if the battery is getting low. Perhaps it could be programmed to bring the boat back to shore while the battery still has enough capacity to make it back!