I think it depends on the type of hatches you are making. I didn't put hatches on my boat, so take my comments with a grain of salt.

1.) I beleive with flush mounted hatches you will want the thinnest kerf possible. I think I remember someone on this board recommending the use of a regular utility (razor blade) knife for cutting out the hatches. If you use a very small amount of pressure and follow the cut numerous times, you can cut through the thin plywood we are talking about. The trick is to take your time and don't press down too hard. Let the knife do the work. This will make a very thin kerf.

2.) I don't remember the plans for the hatches I didn't build on my Mill Creek, but I think they wanted me to cut out an oversized piece of plywood to fit over the smaller hatch opening. If this is the case, then the size of the kerf doesn't matter.

If both these statements are correct, you can get by without a nice ryoba saw.

Hope this helps.