You can also use veneers over the supplied tops. That'll only add 1/32" or so to your thickness. You could also do an onlay as opposed to an inlay. Easier to do and once the epoxy fill coats are applied and sanded it will look like an inlay. 
 

George K

I replaced the ply tank tops and center seat top with a strip pattern made from 1/4" cedar strips and covered on both sides with 4oz glass.  I made the blanks on a flat table then cut to shape using the plywood peices from the kit as a template.  If the tanks are never used as seats, I bet that you could save some weight by going with even thinner wood.

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Thanks GeorgeK, veneers sound like a good idea. 

MarkN, that looks real nice. Did you do something special with the rails? I see the spacered inwhales, but looks like you have a plywood (dark) section, a light section, then another dark section. Looks sharp. 

The center dark strip is just the ply end grain (no rabbits for me).  The outer dark wood is a mahogany cap that is more collision resistant.

Veneers can also be used to cover the pre-installed tops. This will only add about 1/32" to your thickness. Instead of an inlay, you might do an onlay. It's easier to do, plus it'll look like an inlay once the epoxy fill layers are done and sanded.