T&T: battery chargers

[kakillian at att.net]

Can any of you folks help with this one?  A boater next to me installed an automotive battery charger aboard his vessel.  I told him that was a no no but I couldn't explain exactly why, but suggested electrolysis.  What's the real explaination?  He then asked me if it could be used if the battery were isolated from the electrical system on the vessel, I had no answer.  Appreciate the help folk!  Thanks  Ken Killian, Alaska Family Fun, Seward Alaska ps It's not me I have a Lewco.