Thanks guys.
I'm gunna leave it for a while, I think. Finish it off of course (the embankment etc) before thinking of what I'm going to do about the water.
The "mud flats" bit is actually my local soil. Just scooped up some of it from around ant's nests (cause they've already sorted the finer grains out for me) then put it through a tea strainer, then spinkle it on.

(a layer of white wood glue, of course).
The beach area is sloped by about 3/4 inch, again using the ant soil, and then wet down, and dribbled with 50/50 water/glue to get it to set. So if I do put water down, an area of beach will still be exposed.
The "sand" on the beach is actually mosquito coil ash. You know those "green" coil things you burn (smoulder) to fend off the mozzies and bugs? They're green to start with, but the ash is a very very fine sandy coloured powder (like talcum powder). Dusted that on, and I get the sandy bit of beach area.
The rock embankment is 4 Chinese takeaway containers of gravel scooped up from the roadworks where they are re-laying the footpath in town. Put through a course seive to remove the smaller bits and then laid like track ballast. (wet down, then dribble with 50/50 water/glue).
