The water is clear resin. A quick guide:
resin as it goes is not cheap but i have found it to be ideal in realism when i represent calm waters. I paint the bottom of my base with a dark blue/green color (acrylic) and once it dries well enough i apply the resin on top.The resin comes separate with a very small bottle of what is called a catalyst.It basically controls the rate of reaction so if you pour too much the resin gets hard too quickly and vice versa.I think my type of clear resin is recommended a couple of drops on something like half a pint of resin (again different types of resins different guidelines)-mine is casting clear resin for those who want to know-,which i have found too slow(days!!).Besides when it takes so long to hardened the resin, you have a bigger chance of the resin reacting with the painted base and destroying it.It literally pulls apart whatever it is underneath it ,paint, glue, varnish.So i found that by pouring too much catalyst i accelerate the reaction so much that the resin gets hardened within 10-15 min so it doesn't get sufficient time to react with whatever is underneath it ,plus it saves me time on the model itself.Now resin will spread in a uniform manner and it will create an almost perfect glass-like surface.That is not a calm sea exactly, the sea should still have very subtle riddles so(and thats the trickiest part of all) depending how much catalyst and how fast your mix is getting hard ,take a hair drier and once is like a gel start blowing hot air on it.There are ,depending which way the waves go etc, different approaches to this but generally just to create few riddles keep the hair drier directly above for a few seconds at about 10 cm away and with slow motion move around the surface of the base and once you create the riddles take it off. Repeat every few minutes until you are happy with the effect.After that the resin will never go back to the glass like state, the riddles will subside once you take the hair drier off but they will not disappear ,and depending how much you repeat this process the riddles will be more and more frequent and defined.Word of caution:when you put the hair drier for the first time above the resin do it so by approaching slowly from a safe distance while blowing until the resin starts to shake.If you put the hair drier and start blowing from too close from the start you risking pushing/spilling the resin off the base since you don't know how solid or liquid it still is. When the base is hard apply a bit of resin underneath your model and glue it on the water.Do NOT touch the resin with your fingers as even after days it gets hardened it will still be soft enough and with the heat generated from your finger to leave a very clear fingertip mark.Good luck.
...and more techniques for making water: