Suggestion: type "naval war gaming mats" or "maps" into your search engine. When I did, I found several sources (at least five) who can supply realistic seascape mats, at reasonable prices. Respectfully submitted.
find or take a picture of the sea you like and enlarge it to the size you require!
There are large sheets of solid clear wavy fluorescent light diffuser panels that can be underlaid with colored paper. ~1/16” thick sample from our dear departed friend John Eagan https://photos.app.goo.gl/PnheS3t6QjUDxhgA9