Cracks caused by deck flexing.
Did the boat
sail since the deck was put on the new hull?
If not, then look inside to follow the trail to make sure the deck cracks are the source of the light fixture leak. Water
migrates till it finds a propitious spot to come out.
If it were me, I'd want to find someone trustworthy perhaps a licensed, registered surveyor
, who could tell me about the integrity of the deck. And that person will need to be able to see what lies under whatever kind of headliner
your boat has, as well as the deck.
You can seal whatever cracks you find up, but the deck may have itself been damaged by water intrusion. If you tap with a hammer, moist or delaminated bits will sound different from good solid deck (which is a sharper sound).
Sorry to be the bearer of scary tidings.