1) The hole doesn't need to be perfect. It'll have a port on top of it with a 1/2" flange all around.

2) A 5" (or whatever diameter is needed by the port) hole saw will set you back anywhere from $30 - $65. And you will never use it again. The sabre saw on the other hand . . .

3) You don't have access to the back side, so you can't do the trick mentioned above.

4) I forgot to mention the blue painter's tape. Use it. Also keeps the shoe of the saw from marring the gel coat.

Use a compass to draw the circle, cut the hole (you can do a plunge cut in the middle of the cut out or a pilot hole to start), slap the port in with lots of silicone sealant, let cure, go sailing.

I've installed probably 30 ports in the past 36 years - five of them in 17s. It ain't no big deal.