I started using clear silicone caulking over a painted base. That looked nice at the start, but not in the long run. The surface is not glossy enough, collects dust, does not connect with paint, and the stuff shrinks afttr some time.
The next method I tried was casting plaster against wrinkled aluminum foil, which had its merits, but didn´t convince me in the long run. The resulting base is fairly heavy, the process is not fully controllable and corrections are very hard to achieve.
More or less by chance I found that stippling ordinary wall paint with a broad brush will result in a texture similar to a water surface. This is the base for what I have done ever since.