"Sealed" in SLA means that they are sealed against the owner topping them up. They will normally only vent when overcharged, but having vented, they will contain less electrolyte, and their life will be reduced. What they vent is a mixture of hydrogen and oxygen, which is perfectly capable of recombining in an enclosed space, like a hull, given suitable provocation, like a spark. If recharging in the boat, its a good idea to have the lid off, give any hydrogen somewhere to go. Belt and braces thinking, you shouldn't ever need to guard against overcharging and venting, but it saves a lot of potential hassle if you do.
A proper SLA charger with constant voltage and current limiting, and, even better, a means of sensing when it has done enough, will pay its way in not having to buy new batteries.
Yes, Gel Cell and sealed lead acid batteries will vent Hydrogen and oxygen.
Several years ago, we had a submariner "blow up" his submarine twice.
He put his SLA battery into his water tight compartment and sealed the whole lot.
He provided terminals for charging the battery, but did not provide a vent.
When he went to the pond, he launched the submarine and sailed away from the curb.
Upon actuating the ballast controls, the motor brushes arc'd, and the entire boat "blew up".
It cracked his perspex WTC, blew out several seals.
No one knew what had happened, so he went home and repaired everything.
Weeks later when he returned to the pond, the exact same thing happened.
It was then, that he deduced that charging the battery had filled the WTC with
volitile gases, and the the arcs from the ballast motor set them off.
He now has a small hole in the top of his water tight chamber, and he applies the
hose from his vacuum sweeper to it before sealing up the boat.
Charging in the boat will probably be fine, be sure the batteries do not need to be in an
upright position when charging. As long as the boat is well ventilated during and after
charging the batteries, you shouldn't experience any engine room explosions.