There are multiple answers to what is the "best adhesive sealant" for bonded lenses. I prefer BoatLife Life Seal
Gotta say, there is a C&C 35-3 in our club and the owner agonized over having just the right adhesive and he prepped the surfaces "just so" .... and at least one of them leaked anyway.
While we could shrug and say that the structure must flex "too much" in the seas... Naah -- my take on it is that too few owners (and boatyards!) realize that the lens has to ride on a significant bed of adhesive/sealant such that the thermal expansion of the plastic lens will not lose its grip.
I got this advice from an expert boat builder/restorer in 1995 when I replaced the four large unframed lexan lenses on our Olson 34. I put little spacers by each fastening (later found out that small O rings would be been quicker and better) to control the thickness of the sealant bed. Our lenses are adhered to the top of a layer of black sealant about 1/8 to 3/16 high. Yep, it takes several tubes of sealant.
Here we are, 20 years down the watery road, and about time to replace those lenses again due to increasing UV hazing.... and.... NEVER a leak has occurred.
So do consider that difference in expansion between the lens material and your frp boat.
Further... mask off everywhere with 3M blue tape and lots of paper. That sealant will wander all around!
You also know you have to abraid the bedding area of the structure with some 80 grit and clean it with acetone, right? And abraid only the inside edge part of the lens with 80 grit, just a little, to give the sealant something to cling to, right?
Wear old clothes, have a large bucket handy to put paper towels into........
Have a cold six pack of micro brew ready for the end of the project. It will taste all the better with your satisfaction at having saved a ton of $$ at yard rates.
ps: about those "stress" cracks in the window edge gelcoat. I believe that they are caused by slightly too-thick gel coat laid up in the mold... No problem with the boat's engineering. That C&C 35 at our club has them too.
Gel coat continues to "cure" and slightly shrink for years/decades after initial cure. This cosmetic problem occurs on a host of different makes of boat anywhere there is a fairly sharp radius edge.