Hello Rob,
for the water of your loch you can paint the bottom of the water with acrylic paint with green and black (or blue/green and black) you start from the almost black center and you make a gradient towards increasingly lighter green towards the shore. then you spread three layers of acrylic medium to give depth. finally you spread several coats of glitter acrylic varnish with a brush.
it is a simple method, but which limits the risk of failure. I learned about it in a book to create battlefields for Warhammer and it was simple and effective.
best regards.