Not that necessary if they vent into a space that is itself ventilated and does not have
electrical equipment that isn't ignition protect. The point is hydrogen dissipates extrememly fast. Even a very small hole in the top of a
battery box is enough to vent hydrogen. Venting to the atmosphere is best but if not a good option, can be vented to a compartment, that is vented to the atmosphere. It appears that the box is inside a compartment that is part of the living quarters for the
boat (hard to tell from the photo) You could simply snake a hose or tube from the top of the box to the outside. That would do the job.