I do the same as Ed. If you have a wobbly blade, you can tape a piece of thin scrap wood over the point where you are going to do the plunge cut. Transfer the line on to the scrap and plunge through the scrap. This serves to steady the blade as it cuts into the deck.

> I know your fear! I just cut the deck of my CLC 17 LT last night
> (fortunately, all went well). Because I want to use the cutout as the
> hatch cover on a flush hatch, I used a 'plunge' cut with my Bosh jigsaw,
> using a new plywood blade. To do a plunge cut, rest the foot of the saw on
> the deck, with the blade angled up (not contacting the wood). Start the
> saw (fairly high speed if variable trigger) and slowly lower the blade
> until it contacts and starts cutting through the wood. Once through,
> continue cutting in the normal manner. This way you get a clean cut only
> as thick as the saw kerf, and don't need to drill any pilot holes.
> WARNING: this was on a plywood deck that was not fiberglassed. But I don't
> think cutting through glass will present much of a problem. Good luck!--
> Ed