The link gives no indication of the operating principle, but my guess is it works on the Peltier Effect. What you want is a device that uses the related Seebeck Effect.
And these are readily available, and are called thermocouples. There are very common as a temperature sensor. I have one to go with my multimeter.
The bad news is, you get very little voltage out, though the current can be enough to be useful. A unit with many in series (sometimes called a thermopile) is used in gas central heating boilers - it works as a flame failure device and holds the gas valve energised while the pilot flame is present and heating the thermopile. If the flame goes out, the valve de-energises preventing an escape of gas.
So without even doing the maths, my guess is it would be impractical to extract enough power to be useful, given that the water is very low grade heat as compared to a flame.