Garage belly board needs fixing
Seeping moisture or leaking from cracks, either one is not good. Yes, sealing and or patching will be much easier but it's something you'd have to do every year as the problem gets worse. If those boards have been exposed to moisture over the years, they are probably starting to rot from the insides.
Give them a good pressure cleaning and check them out more thoroughly. If  they appear to be in good condition, I'd think about covering them with one of the many roofing membranes that are out there. If they show signs of rotting, then starting to replace them will be your best bet.