A word of caution, when I installed mine 5 years ago, drilling out the old rivets was a pain... They spun and just burned a hole through the plastic. Once I got the port out, I realized that the fiberglass around the edges of the port was brittle. I put the new rivets in, but I'm sure that some of them didn't hold very well because there wasn't much for them to grip against. (Could also be the source of my leak) I applied the silicone liberally to the rivet holes and exposed fiberglass, as well as coating the underside of the flange. I put in 5" inspection ports in the aft deck a year later, and this time used 4000 sealant instead, which I think has sealed much better than the silicone. It's an adhesive, not just a sealant, and the 4000 variant is UV resistant as well. If you buy a tube of black pigment and mix it in before applying, you can even color-match it to your hull (my hobie, though "white" in color, is actually somewhat grey). I didn't even bother with rivets or screws, mostly because I bought flat deck ports and didn't want to warp them. (Purposely bought the flat ones, so I could use bottleports in them:
http://www.bottleport.com/)
Five years later, my Hobie supplied white ABS plastic deck ports have yellowed horribly. The 5" grey polypropylene deck ports I installed in the sterns, on the other hand, still look grey. If I had to do it again, I'd buy the grey polypropylene deck ports and use 4000 to seal them, with machine screws, nuts and washers rather than rivets as a secondary, mechanical means of securing them to the deck.
*Edited to correct the type of 3M sealant used. 4000 sealant is the type I used, UV resistant for deck fittings. 5200 is more for sealing/adhering through-hull fittings.