I built my Mill Creek with 1/4 inch luan. It worked fine for the hull. Each panel of the 5 panel hull had only very gentle twist, so the thicker luan had no trouble. The plans called for glassing the hull anyway, which added lots of extra strength to the luan. However, when it came time to put the deck on, the 1/4 inch luan would NOT bend to the tight radius needed, so I ended up buying the 3mm marine plywood for the deck. I imagine that 1/8 inch birch plywood would work for the deck also.

One trouble I did have with the luan was that one of my scarf joints broke while handling the panels in the shop. The break was not along the glue line, but was in the wood next to the glue line. The break started at the scarf joint where the thin outer ply had been severed, it proceeded through the middle ply, simply splitting that ply along the grain, and it stopped at the opposite outer ply where the grain went the opposite direction again. I put some epoxy into the break, and reinforced it (on the inside of the cockpit) with some glass tape. The joint is supported in the finished kayak by the neighboring panels, and the whole thing is wrapped in cloth on the outside (as per plans,) so it is plenty strong. My only mistake was being too rough with the panels and not holding them on edge when moving them around during assembly.

Marine grade plywood is better if you can afford it, but it is not neccesary.

Hope this helps.
-Dan