To go with my 4′ x 4′ port boards, I needed a 2′ x 4′ section of water. Rather than just paint a bit of board, I thought I would life difficult for myself and try something different.
I did start by just painting a bit of hardboard various shades of blue, perhaps not dark enough, but as a first go it will do.
Once that was all dry, I spread a whole tube of silicon sealant over the entire surface with a wooden spoon. This left an interesting texture and helped to distort the paint underneath.
There are a few small gaps where the silicon missed here and there, but that sort of adds to the effect. There are also a few bits where the spreading has left an undesirable mark, but I think I can do a better job on the next version.