foam board for ship dios

Pink foam insulation board available from Lowe's/Home Despot.   4'x8'x1" sheets.   I have seen hobby/craft versions cut to 2x2 on the same insulation aisle

There are two basic types.

Foam bead board is the cheap stuff. It is only useful as the most basic of materials because it can't be carved without crumbling.

Extruded board is the expensive stuff. It's either blue or pink. It never gets used for packing so usually you are forced to buy it.

To expand on Bill's explanation, both the pink and blue boards use a closed cell foam (finer than the open-cell foam in ordinary foam.

The pink is used in ground contact where high compression strength is required--it crushes at 3200psi.  The blue is more for wall insulation and crushes at only 1200psi or so.

As a rule of thumb, the pink is more expensive, but the grain is a bit finer, which can make carving slightly nicer.  Now, telling the difference between the pink and the blue is more than passing difficult for a modeler.

Nicely, hot wire cutters make working with either foam very pleasant.  They take regular (PVA) glues nicely   Polyeurethane glues (aka "Gorilla") can be used, too, but you need weights or wide boards and clamps as that glue expands.

Now, all of the above presumes you are in the US or Canada, in other parts of the world, these foams are refered to by a plethora of other names (Polystyrene in some places)  Some of those foams are slightly different chemistries, too.

If you are intending to model water surfaces, you will want a much smoother surface to work upon than the bare foam.  Generally a coat of plaster or PVA-soaked tissue paper, then coated in gesso or gel medium, or any of a dozen different techniques.

Doesn't somebody make Plexiglass in a few different colors with a water effect?  It wouild be useful for ships underway or subs under water.  

If you don't mind a heavier material, I often use particle board.  My local building materials store (Menards) often has vinyl laminated particle board shelves on sale at bargain prices.  Typically they are about seven or eight bucks for a 1 foot by 4 foot shelves- that often makes me several bases.

I often used the pink board on my n scale layout. At Home Depot I often found it sold in 4x4 squares.

I've tried several materials and prefer the hard Floracraft Styrofoam found at craft stores. It can be found in 1'x3'x2" sheets. It's non flexable but easy to form, easy to cut and makes less of a mess. The blue and pink stuff make a serious mess when worked with IMO. A water colored plexiglass would just look silly and distract from all the work you put into the kit. If you want a clam seascape, use resin.

I 2nd MC's comment about pink foam making a mess! Dang it goes everywas well. Took forever to clean up and i just threw away the wire wheel i used to dig out the waves.