I did one S & G with 1/8" Luan and it worked out fine. $8 a sheet. I'm working on a 2nd S & G right now and I'm using some left over Okoume and some 1/8 Luan. There's not doubt that the Okoume is better quality - no voids and better wood veneers in the make up of the plywood. The Luan is stiffer and actually feels stronger. The Oloume bends easier and I'm using that on the cockpit area of the kayak where it has to bend more. Bottom line is: I think you can build a fine boat with 1/8 luan. True marine grade plywood is also expensive and I'm not sure it comes in 1/8" thickness. 1/4 could get heavy and depending on the design, may be tough to bend.