I'd use a good quality ('name brand'), new wood-cutting blade. It will be dull once you cut the opening if the deck has been glassed.
Something about 10 teeth/inch (tpi) or finer.
Some jigsaws have adjustments which affect the orbital action; check your manual for the setting for smooth (vs. rapid) cuts.
I don't worry about narrow kerf, since I've found that I always need to sand the cut edges to smooth them out and also to provide enough space for epoxy (and varnish) on the edges. You don't want to get a hatch lid jammed in the opening, and the waterproof seal doesn't depend on tight edges.
If you have some scraps, you could try the blade on those before cutting into the kayak. Start the cut with a line of (minimum size) drilled holes which you can join with some back and forth action with the drill - not the 'approved technique' but it works for me.
Something like this would be what I'd grab first:
In some ways I can be a very slow learner. It took me quite a few years to realize that a good quality jigsaw wouldn't produce the results I expected with a 50-cent blade from the no-name (or store name, Mastercraft) 'assortment' package.
Bosch (I like their pro-grade barrel jigsaws) or sometimes Makita blades for me, now.
Years ago I bought a lot from Jamestown Distributors - semi-annual trips for sailboat build supplies (bronze fasteners, etc) and Sterling Glass-Fab blades were usually on my list. They are excellent.
I used the same Bosch ones John showed. We traced our pattern on masking tape. One thing we learned was to always have the tape joints overlapping from high to low in the direction you will be cutting so that the jig saw doesn't hang up on the shoulder of the next piece of tape.