JU-88
There are quite a few ways to model water.
Probably the easiest to use would be Woodland Scenics Realistic Water. Just pour on to form the main body of water and then use their Water Effects to make ripples or waves. But it is quite expensive.
Another method is to use clear casting resin. You will probably get this at an art store. I have not used it but it will give a nice clear finish or you can colour it. Bad fumes so use this stuff outside.
At the moment I am doing a stream in a diorama and using artist's acrylic medium.
Sometimes called acrylic gel. Get this from an art store as well. It dries just short of clear and can be coloured with paint if needed.
I guess the best advice is that you will need to practice first with whichever of these you use.
Have fun.