There are two "lenses" on that deadlight, an outer and an inner. The old one outside is thin, and prone to cracking. these are just glued in. Deadlight/skykight replacement kits are available.
Take your old one out, clean away all excess old adhesive/silicone residue, lightly sand bedding surface, and clean with acetone. Tape around the edges of the hole to prevent excessive squeeze out. Tape around the top edges of the new outer lense for the same reason. lay a bead of adhesive, (3M4000 or 4200 comes to mind) in the landing of the hole, and set the lens in place. Wipe up and clean any excess that somehow got any where not already taped. If you are doing the repair, outdoors, you probably want to remove the tape at this point, to prevent sunlight from attacking the tape and leaving gummy/gooey residue. Wait for it to cure.
After a few days, reinstall masking tape all around again, and on the face of the lense to the edge. Apply another application of sealent, working it into any gaps,filling and smoothing the gap between the lense and the boat's deck. At this point you are doing cosmetic work; try to make a nice smooth flat shiny bead all around the edge. When you are satisfied, VERY CAREFULLY remove the tape, and leave to cure for a few more days.
On L.L.attitudes, I decided to leave the stock inner lens in palce. I have a spare as a result.
Good luck!
