Yes that is the biggest problem with the river tiles. The water doesn't look right. I will use a mixture of ready mixed filler PVA and blue paint to get rid of the texture of the polystyrene . Then different shades of blue and green , then gloss varnish ( I have one that doesn't dissolve styrene) .
I won't go for the commercial water effects as the project is so big it would get to expensive quickly. There are another 7 river tiles, 4 beach tiles and 4 water tiles to do .