Is use of a deep cycle marine battery indoors safe?
3% hydrogen to air is explosive. Even sealed batteries discharge some hydrogen gas when charged--they have to release the pressure or they will break the case. The seal is accomplished with a rubber (or silicone like material) over the port and usually a plastic shield welded in place to keep the seals in place. These batteries are not designed to be refilled.

The risk of explosion is very low with both battery types unless overcharged. Open cell or vented batteries require a flame arrestor (see NEC 480.11). In some systems this is as simple as a strip of mesh fabric like red  scotch bright secured over the cells it needs to be acid resistant because a large component of batteries is sulfuric acid.

With the flame arrestor in place both batteries present similar hazards with the exception that marine deep cycle batteries many times end up being larger (more amp hours) so they take longer to become over charged, so there is really no difference in hazard after a flame arrestor is installed.