The only difference between marine ply and normal ply is the glue used on the lamination of the plys.
Most ply laminations nowdays are using glues that are almost identical,in sonme instances they are the very same.
If you get ply for shutting you will be getting the very worst, but getting normal exterior ply should be fine.
Now being as the boat is not full sized and will NOT be kept in water, you probably will never know the difference.
Also marine ply is often used with inferior grades of ply as it will often be covered by paint or something similar.
This is one of many reasons the grain is sometimes good on one side and very poor on the other.
If the wood is sealed good you wont ever notice a difference especially on a model boat.
Shakespeare powerboats used marine ply for their 27 foot powerboats during the 60's and early 1970's.
But not on their speedboats, the speedboats that were made by this company are often still seen around to day.
Bristol wooden powerbioats didnt use marine ply neither, this could of been to do with weight i dont know.
When full size boats need to have ply replaced they do use marine ply, this is because water has already attacked the
beams and insides of the boat etc and marine ply gives that little extra help.
But even with marine ply, you still need to seal the wood, this can be done with eithera 50% varnish and 50% white spirit mix
or with a epoxy coating, the choice is yours.
Riva speedboats seal their boats using the 50-50 varnish and white spirit mix, and if its goiod enough for Riva, then its definitely
good enough for model use.
Other companies who have used this method to seal wood in the past has been c.j broom norfolk,peter freebody hurley,
w bates and son chertsey,hucks boatyard hampton,turks boatyard teddington to name just a few top notch boat builders.
If you dont seal the wood then paint or varnish can come off leaving exposed wood that will soak up water and eventualy it will
come through and damage the interior of the ply.
Also by sealing the wood you will not see as much movement in the wood due to temperature and moisture.
By the process of the sealing, the fibres of wood change, so whatever wood you choose ,do make sure you seal it.
why not just use birch ply for the whole outside of the boat anyway ?
youl find it gives a much smoother base to start with.
if you were going to leave the boat in the water all the time, then circlip would be right
phill