Water will take the path of least resistance and for me I have seen it mess up a few thing on my 1988 Mark II. An example is water weeping down through the closest starboard stanchion where the electrical panel is located inside. The moisture slowly dripped down, attacked and disintegrated the wire sheathing covering the line to the hot water heater breaker. It corroded the wire after making its way through the cover (became brittle and cracked open) shutting down the heater. The breaker kept shutting down! Water also made it way through a stanchion mount on the port side near the V-Berth, impacting the balsa core which bleed down the inside of the boat. Lets face it, Catalina did not in my opinion do a great job on the stanchion setup. No backing plates, flimsy mounting along a flimsier gunnel that all eventually require rebedding (how about that butyl rubber). So water made its way following your light wiring and filled up the fixture. Simple fix: look for the closest mounted anything to the light fixture (teak handrail, stanchion, mast mount, cleat, port, etc) and start there. You could do a dye test by adding a few drops of food coloring to a glass of water and pouring it over the closest thru deck anything close to the fixture. Keep the gravity "downhill flow" in mind. Its best to attack the point of original entry rather then seal the light wiring hole. I think as a rule the stanchions should be removed and rebedded every so many years with the Catalina 30. Heck, so should everything else attached to the deck!
Bob
Lake Lanier, GA