Quite simple construction really, hardboard base with 25mm insulation foam layers. You've heard it called 'blue' or 'pink' foam but this stuff is actually a strange mustard colour :-/
The field borders were made from 2mm thick card cut with a bevelled edge and filled smooth with normal household filler. The whole lot was given a quick sand down to get rid of the worst lumps and then everything apart from the fields themselves were sanded as normal.
The fields themselves were painted with a very dark green and highlighted around the edges and corners. They were then given a coat of yacht varnish and will be given more when the first coat has gone off. The more coats you can put on the better as it really adds to the depth of the water.
At some point in the near future I shall get my boards out and do a sort of photoshoot but enough yammering form me, more photos below :-)