I have one slight concern with this project, it all started by using a very small sized model CAD file the boat was about match box size the wall where very thin, but after scaling up ended up a tad on the thick side , although not solid has a skin over a honeycombed core I feel the vessel could end up top heavy. I guess only time will tell.
As with all I do .. Always a 'Plan B' Col

Thanks once again for all the help with my project Col

He Col!
I love the Polyethylene foam packing material that comes in many electronic shipping boxes. It's designed for cushioning and is not rigid like styrofoam. It's easy to cut, doesn't create millions of loose styrofoam dust pellets when you cut it, is malleable, and stays in place.
😉

Table tennis balls work well I am told by the Buxton model boat club I bought a box of 50 for very little money and glue them in with a hot glue gun.

I have never used any and still have all of them.
I suggest when the model is finished you do the 'Upside down test'. As it says turn the model upside down and see what falls out and then go back and secure it.
Roy