Re: Making a Bench Seat (Frames, Foam & Upholstery) w/Videos
First let me qualify my input by saying I know nothing, but I did stay at a holiday on last night! But...I would not use those type of fasteners. I have 6 Solid Oak dining room chairs that use these to attach legs, seats and back...in the first year of use they were all so loose I had to pull them out and use a different anchoring system. I used expanding concrete inserts (also, would not use these for a boat, as they are not stainless) Which have lasted longer than the thread in inserts, but still not perfect.
I think your idea is awesome and sound; however, I'm afraid these threaded inserts mounted in plywood will have much less grabbing/staying power than the solid oak in my dining chairs, especially with the vibrations from a boat! The inserts (maybe only one will do this?) will eventually "egg-out" their holes and cause the insert to loosen, not to mention introducing moisture to the equation, which if the bolts freeze will be difficult to screw in/out then you'll have to pull them through your nice new floor!
My rock and a stick method...I would use epoxied plywood with stainless "T" nuts on the existing deck that way there's no penetration through the deck (see attached horrible sketch). Not as clean nor as industrious, but IMO, a easier/more secure way to keep your seats in.
I'm looking for a better anchor system...