The easiest way is to copy whats there. If you want something custom, look at as many boat interior pics as possible. Find a style you like and start. If you plan on seperate pieces that touch, make sure you allow for foam/vinyl. Its a good idea to keep seat cushions tight. As far as the wood goes...3/8-1/2 depending on how strong you want em...I went for 1/2 ply. Used wood glue and a brad nailer for assembly. I then used fiberglass/resin on inside corners and areas needing reinforcement. I then sealed all wood with a good outdoor sealer. Talk with your upholstery shop as per tolerances. Building a interior is very simple, just take your time as it will show.