Testing used batteries

I need some technical advice here.

I have collected some used Golf Cart Batteries for an all electric boat being put together.

These batteries are the same size as a fat Car Battery - but are deep cycle, heve and 6 volts - so two in series gives me the required 12 volts.

At this stage I only have my old 4 amp car charger.


Anyway, I wanted to see if any of these batteries were any good.

First I checked them with a hydrometer and got a mixed result. I tope dup the water and left them for a week.

Today I got around to setting them up on a wooden base, hooking them up in pairs, and started to charge. All of the pairs showed up on the charger as “ full” - 4 red lights.

But on the hydrometer they are showing up as Red.

Anyway – how can I successfully check if these batterys are worthwhile or they are just ballast?

Thanks

Gary