The space between the deck and the coaming is 18 mm (about 3/4 inch).  I ran into a similar issue with the Wood Duck I was building from plans; the plywood was 4mm.  I decided there was no way I was going to cut out 4-5 of the thin 4mm coaming spacers and stack them up for the coaming spacer.  I could have found some 9 mm plywood but I didn't have any.  However, I did have a bunch of bead and cove cedar strips lying around so I used them to make the spacer (for some reason I'm thinking you might have some leftover from the hybrid deck of your Shearwater too!)

I used a chop saw to cut the strips into 3/4 inch long pieces and then placed them vertically around the cockpit opening.  Since I had a pre-cut cockpit rim with an inside diameter the same as the cockpit opening I'd already cut in the deck, I placed the strips on top of the deck, using hot glue between each strip piece and the deck and Titebond in the bead/cove joint between strips.  It only took about 45 minutes to do the entire (huge) Wood Duck cockpit and it was lots of fun.  Then I used thickened epoxy to glue on the cockpit rim on top of the vertical strips, making a nice fillet between the deck and strips and between the strips and rim (on the outside of the coaming) to help structurally.  I applied bias-cut fiberglass strips both on the outside (top of deck-coaming rim-underside of coaming rim) and on the inside of the coaming (wrapped from under deck-inside coaming rim-top of coaming rim). 

I wasn't sure how strong this would be, but it's held up really well, looks good, and is very sturdy. It's another option to consider in case you don't want to cut lots of skinny coaming riser pieces from plywood or purchase 9 mm plywood. 

Photo sequence and more discussion here, about 3/4 way down page:  https://picasaweb.google.com/113605413461236703013/WoodDuck12Construction