Ignatieff, the best material that I've found for playing surfaces, sea or otherwise, is faux suede upholstery cloth. It's thin, backed by cloth, lies flat, doesn't need to be treated, can be washed, is easy to cut, isn't too expensive, and comes in some very natural colors. For sea battles I use a color that looks very much like the "Coppen" shade on this web site:
https://www.onlinefabricstore.net/product-group-faux-suede.aspx?application=upholstery&type=fabric&gclid=cl-12a738swcfvbbfgod-38jtqOf course, if you are doing WW2 night surface battles, you might want to use a darker "navy blue" shade.
For islands I just use small hills, which can be plastic, resin, or made from dense styrofoam. When I want a beach or atoll I use pieces of faux suede upholstery cloth in sand colors (anything from brown to nearly white). If you want the island to look like it's covered in jungle, one can just glue on a mass of craft store moss or other greenery. At 1:2400 scale it doesn't have to be detailed.
I've been checking the web and I see that there are several suppliers of faux suede upholstery cloth in the UK.