I did this project a couple years ago, following the procedure demonstrated video (link below). My portlights leaked and a PO had made a mess using hardware store silicone, which was a pain to remove.

I also used 3m vhb (very high bond) tape and Dow 795 as mentioned by njlarry (and in video link provided below). I was very pleased with the outcome. No more leaks and looks great. My portlights are tinted very dark and had a few little nicks and imperfections along the edges so by using black 795 the result was a clean and professional look. Good luck and take your time. Like a lot of boat projects its all about adequate preparation and using the correct materials.