I just started with cardboard modelling with Chunky Dungeons.
As I could not find foamcore here in germany, I used cork sheets. They are used as floor boarding or for pin boards and you can get them at a hardware store for about €6 for a sheet of 900 mm x 600 mm x 5mm (about 35"x 23"x 1/5") or about €10 for 12mm (1/2") .
They can be cut quite easily with a hobby knife and are elastic yet stiff enough to not warp. Uhu glue works great on them and the thin one is good for base tiles while the thicker one is stiff enough to be used as a masterboard.

For other terrain I have used Masonite (patricle board) wit good results.

A friend uses vinyl floor tiles.

For the Maiden I might use a particle board base painted sea green then covered with epoxy resin, wait until it starts to set, then use a broad screwdriver, back of a spoon, whatever, to add ripples to the surface. you can mix a few streaks of a darker blue green in with the epoxy.

The Auld Grump

*EDIT* Another possibility for the Maiden might be to use the particle board and a piece of the plastic they are selling at Michael's craft stores to simulate water for their Christmas village sets. Paint the Masonite a darke blue-green, then glue the vinyl 'water' to it. I am using a varient of this for my ghost maiden if I ever get started on her. (Just finishing the regular Maiden this week coming... it only took me what, six months? the Maiden is a model I wanted to take my time with.)