Get a long sheet of paper - either backing paper or lots of sheets fastened together. Paint it with
salt paint and allow to dry. Add a sandy bottom - either spread pva glue and sprinkle with sand or cut up sandpaper (if your teeth will stand it!). Attach strips of seaweed - crepe paper, coloured paper, tissue, ribbon - add a few twists for texture. Get the children to draw, colour, paint/sponge or make fish, starfish and seahorses and stick onto the mural. Don't forget woolly tassels - they make great octopusses (octupi?!?) and anemones.
You could even have a treasure chest with an opening lid - hide some gold and silver foil coins under the lid for a lovely surprise!