Bad battery or charger gone bad

Went into the garage this morning and it smelled like rotten eggs...knew it was one of my Skeeter boat batteries. Minnkota battery charger had a red light on battery one. Filled cells with water, plugged charger in. Checked a couple hours later sounded like water was boiling and started smelling like eggs again. Boat is only two years old....batteries are Continental. I know I need to test battery....but what is the likelihood the charger has gone bad and is over charging? Thanks!

Low Water caused it to Sulfate. Replace Battery.

My guess is its not the charger if you put water it the battery it may be to far gone if they get to low they will short out between the plates . How much water did you put in it? If I'm wrong some one will be along here in a bit and tell you.

You over charged and fluid has came out of the battery...they may not hold a charge as well now...oh I missed the adding water part..probably bad batteries...

Get a good battery hydrometer at the auto parts store, about $10 I think. Don't get the tiny cheapie one.
Remove caps from battery, squeeze bulb to expel air, insert tube into electrolyte and slowly draw it into hydrometer taking care not to cause bubbles that will mess up your reading.
After taking a reading for a cell, slowly squirt electrolyte back into the same cell.
Move to another cell and repeat, but be sure to squeeze bulb an couple of times to "rinse" the prior electrolyte residual from the hydrometer. THEN take a new reading.
Often, a bad cell will make itself very easily identified by comparing the 6 cells of a battery. One or more are usually much different than the rest on a bad battery.

The above testing will probably make it clear if you have let the battery go too far. Boiling down to expose plates is a big no-no that will cost you.

BTW, all the multi-bank chargers I've dealt with have dedicated output stages, so it's easy to find out if a bank isn't working correctly. Swap BOTH (+ and -) leads with the charger leads on another battery. If the problem moves to the other battery, the charger is at fault. If the same battery has whatever problem ails you, then the battery is at fault.