30 ports in 30 years Man that hole saw would have really paid off they were cheap thirty years ago! :-)

Don't get me wrong jig saws are ok not my first choice. But a saw is a saw be it hand, air, jig, or hack. Make the hole and goop it up.

I'd stay away from using 5200 marine silicon it will bring up half of your deck if you need to replace the porthole. Designs of portholes change and the lid you have now may not be found in the future experience speaking here.

Get some fiberglass dust on your arms and go sailing. Got to love it!