I suppose it would depend on the plywood and how stiff or strong it is, but I don't think I would go thinner than 3/4". If you make foils (and especially their leading edges) too thin in the interest of reduced drag, etc., they often don't really behave very well and can tend to want to wander side-to-side, rather than just cutting through the water and being done with it (how's that for a scientific explanation?).
In general though, I pretty much detest everything about plywood leeboards (and rudder blades). Only about half of their wood fibers are pointing in the direction you want for strength and stiffness, the rest are mainly just along for the ride. Vulnerability to water intrusion (and eventual damage from it) is greater than on a regular wooden slab if the leading edge or tip get dinged-up from rocks because you have exposed end grain on all sides and I don't think they look as nice as real wood. I'd much rather get a couple nice hunks of 5/4 mahogany or other nice wood and spend a little time with the spokeshave rounding the leading edge and tapering the trailing edge. There are just some things that look rather cheap in plywood and rather elegant in varnished wood and I think leeboards and rudder blades are among them - plus they usually work better. Option #3 would be laminating front-to-back with smaller slabs, which can also make a nice board.
You can spend as much time as you feel you want to devote to making them a good airfoil shape, but in the grand scheme of things, the rewards for doing this may be fairly small and take a GPS or knotmeter to really see. Thousands of boats (like Sunfish and similar production designs) have sailed a lot of fun, successful miles with foils that were nothing more than wooden slabs with rounded leading edges and tapered trailing edges.
