I am not sure about your questions, because I am not sure if I am addressing what you mean by "what is water constructed from"...But this is how water got here.
Note this is a recent theory, I found it on the National Geographic Channel, but I have'nt found many other resources that back it up:
Water, to simply put it came from outer space
....I sh!t you not.
Basically when the earth was in formation from its 'Big Bang,' it was too hot to carry any water. Due to it just being formed and from it being pulverized by asteroids , it was simply a ball of rotating hot / molten rock...any water would have been vaporized.
Also, Earth is one of the 'inner planets' (not to quote Sailor Moon), the 'inner planets,' in that it is in between the asteriod belt and the Sun. The inner planets include Mercury, Venus, Earth and Mars, and right outside is the asteroid belt and its partner, Jupiter. The inner planets were generally too hot to salvage its own water, but outside the belt it laid frozen. So basically the asteroids in the belt held frozen water, frozen for billions of years, and with Jupiter's huge size, they (at times) would be knocked out of orbit and sent straight to Earth.
Over many many years, we eventually got hit with so much frozen water, we got our seas.
Again, this is a theory recently proven...even I find it a bit skeptical
But my source does explain that:
In January 18, 2000, asteriods lit up Canada's sky as they were then pelted into frozen water, they were then dug up and sent to a NASA lab. We found out that some of these rocks were made up of 20% of water. And if the earth was constanly pounded by thse asteroids, that in the course of less then 150 million years, we got our seas
...Go figure