That would be depron- which has become (more-or-less) the standard material for r/c planes
Depron is expanded polystyrene (eps) foam which has a thin skin on each side of the sheet - if you cut you own sheets of eps it does not have the skin
depron available from 0.5mm ro about 12mm thickness - naturally the thinner the sheet the stiffer and heavier (more skin, less foam)
Foamboard is effectively depron which has paper bonded to both sides.
BTW - it can be gently bent if the paper is removed or softened (soak with warm water) and will hold the bend
As you say it can be waterproofed with acrylic or polyurethane - or emulsion paint, or acrylic paints etc
andrew