This is the start of a long-overdue Plascore page. It is possible to build any stitch and glue driftboat with a plywood bottom. I used to use 3/8" AA Marine fir, glassed with 4 layers outside and 3 layers inside. Many more have used 1/2" plywood (or its mahogany equivalent measured in mm)
But polyethelene honeycomb core sheets are available now, with brand names like Plascore and Nidacore. I only use honeycomb core now. It's better stuff. If you ding the fiberglass skin in a plywood bottom you have to patch it right away. If not, you risk saturating the plywood with moisture.
If you ding the fiberglass skin over a honeycomb core bottom you can usually wait until the end of the season before patching. And it is lighter too. You will have to add some additional fiberglass.
Two layers of 10oz glass on the bottom, end to end, with an additional two more layers sandwiched between the full-length layers, in the middle impact area are a minimum. And two end-to-end layers (of 10oz or perhaps 6oz) on the inside, with perhaps one an additional shorter layer (perhaps 8 feet long) sandwiched between the two end to end layers, centered on the impact area (inside), are a minimum. In this case I recommend more glass on the outside, largely for abrasion reasons. More glass on the inside is what you need to protect against implosion from big impacts. Still. Three layers inside is usually OK.
Call the 800 number for Plascore and/or Nidacore and ask for a delivered price on 3/4" panels that include "a resin barrier." The salesman will know what that means. You can get honeycomb core in various widths (4', 5' etc) in any length. So, if you get a 5' x 16' panel you don't have to join it. Shipping is more expensive for large panels however. So I usually get 4' x 8' sheets and join them together with glue. Once joined, you can then treat the resulting panel just like plywood.
Joining Honey Comb CorePlacore is not scarfed. You simply cut 8" x 8" inter-locking blocks into the joining edges....square on the sides or (if you want) dove-tailed. Then place the panels over visqueen, over a large flat surface (perhaps chip board over long 2x4s over saw horses). Wet out and putty the edges. Try to minimize putty squeezeout to almost nothing. Subsequent glassing steps will provide the real strength at the joints.
Push the panels together. Cover the joints with a strip of visqueen. Cover the visqueen strip on top with a strip of plywood. Drive drywall screws through the plywood, through the honeycomb core, into the chip board table below, in order to line the edges up vertically and to hold everything firmly together while the resin-putty sets.
That's it. From that point on you can think of the honeycomb core panel as funny-looking plywood.