You don't say what brand or type - so I am going to make some assumptions. There are several steps 1) Confirm the manufacturer- on the Catalina it may be Lewmar. 2) if so, the older design had a two piece exterior aluminum frame with a rubber gasket at the 3 o'clock and 9 o'clock position. (My boat was so equipped and the rubber spacer was completely shot) 3) If so, consider digging out the rubber and cleaning well. Tape aluminum on each side of the gap, Use a glazing sealant like Dow 795, (this, and others, is a glazing silicone that is different to work with. It still gets everywhere if you are not careful and is tedious to clean up. It takes 24 hours + to cure) work a small amount on to the aluminum surfaces to assure adhesion. Finish caulk the joint and scrape so the tape is clean with a dragging razor blade, wait about 30 minutes and remove tape - this should get you a few years and is easy to do.
If leaking between the frame and the fiberglass, you will likely have to remove the portlight. You can use a sharp razor knife carefully to not create any fiberglass damage. Others have used a piece of piano wire with two wooden dowels for handles. Once the portlight is out, you have to clean ALL old sealant/silicone from the frame and the fiberglass. 3M has a 1/2" wide double stick bedding tape for reseating. You hold the portlight in position and trace the outline. The tape should be inboard of this line - 1/8" to 3/16". Test fit with the tape release paper still on. Once satisfied, you will need TWO people to exactly place the portlight because it is a once only event or start over. I believe you have to brace the portlight against the tape for a period of time (refer to mfg. instructions). Tape the fiberglass and the frame with blue tape. Caulk to exterior with Dow 795 or equal (run-of-the-mill silicone is NOT recommended).
If the lens/frame seal is shot, you can buy a new seal and install in no time- with Lewmar the seal looks like it will go in either direction but follow the mfg. directions for correct placement.
Good Luck