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Removing the surface charge is typically done on a car by leaving the headlamps (10A load) on for 3-5 minutes. A larger boat battery will need a larger load, but just hitting everything on the panel for 5 minutes should do it.
Then you still have a false surface depletion voltage showing.
It can take 24 hours for the electrolyte to circulate (remember, there's no pump in there) and even out the charge in a wet cell. A false high voltage will show for at least that long.
12.6-12.8 volts is the maximum any nominal 12v lead-acid battery will ever charge up to, because of the chemistry of the battery. (Which varies a bit from brand to brand.) Anything over 12.8 is an illusion from surface charge, and every battery maker will tell you the same thing. That's also why their warranty test consists of using a LOAD TESTER that pulls high power form the battery--and not doing a simple voltage test. You'll find load testers on sale for $25-100 in "automotive" and "truck" sizes. They'll pull a thousand watts from the battery, emulating a starter motor's load.