If something leaks on a boat (or a car or a house or literally anything) never plug the leak from the inside! Quoting Taylor Swift (to whom we can look for advice on all manner of topics):
Like … ever.
Always rebed a leaking item by completely removing it, stripping away the old caulk, and re installing it with new sealant. This sounds like common sense, doesn’t t? The world’s uncommon commodity.
Aboard our Sabre 42 I have noticed thick beads of caulk on the inside of each deck hatch frame as seen in this photo:
Since we’ve owned the boat we’ve been noticing water entering the forward head and wreaking minor havoc ranging from fatally corroding a ceiling light fixture to generating black mold in the back of one of the storage lockers. After re sealing a variety of items in search of the source I finally decided to pull out the head’s deck hatch.
Sure enough: at some point in the past a prior owner had gotten a minor unwanted shower at the hands of the hatch and, instead of removing and rebedding the hatch as common sense (and pride in one’s work) would dictate, s/he decided to inject a bead of sealant around the hatch frame from below. This kind of cheap shortcut will always end with the same result: water will continue leaking into the boat but will migrate along the inside of the headliner to a new location. It will cause at least as much mischief only now will be be doubly troublesome owing to its throughly concealed original entry point.
In the photos below, note the dried out and discolored sealing caulk at the deck level, and remaining traces of the pristine white caulk that had been deployed from below.
The obvious difference in their age tells the whole story. Ugh. This poor, incompetent work tripled the level of effort to fix the leak relative to having fixed it correctly the first time – especially considering the additional need to replace a destroyed light fitting and to remediate black mold.