Agree with the above... acrylic (heavy smoked - looks best from outside and still provides plenty of light below) and DC 795 works very well. We've used it for 'screw-on' applications as well as in-frame windows with good results.
1984 Fast/Nicholson 345 "FastForward"
".. there is much you could do at sea with common sense.. and very little you could do without it.."
Capt G E Ericson (from "The Cruel Sea" by Nicholas Monsarrat)