Tender...?
Charger or tender?.... If you buy a tender, you can leave it in the boat. The tender only delivers the amps needed to maintain a full charge, then it shuts itself off.
A charger on the other hand, will continue to run till the battery has a full charge. Once the battery has a full charge, if you continue to charge, the battery liquids will begin to vaporize. This vapor is not water, it's hydrogen sulfide. Very explosive.
You can use a tender while connected to our electrical system, you must remove the negative cable to charge with a charger. If your using a battery charger, I also recommend you leave the engine compartment hood/canopy cracked open so any gas build up can escape, decreasing the likelyhood of an explosion............:cheers: