how are bridges built on top of water

Floating bridges r impractical in most locations because of diurnal (or semi-diurnal in some locations) tidal changes in water level. There r some floating bridges over lakes that have closely controlled water levels using locks or dams like Lake Washington in Seattle which has 2 very long floating bridges across it. But the US Army Corps of Engineers carefully maintain the water levels of Lake Washington using locks in Ballard.

But most longer span bridges use very large concrete support bases that r made using hydraulic cement concrete. That is a type of concrete that will harden underwater. These very large structures r made by placing a digging through the sea floor till bedrock is reached using a watertight caisson on the sea floor. The Roeblings made this method famous during the construction of the Brooklyn bridge. It was also when ''the bends'' was first discovered, because the workers were working below sea level for many hours.

But u r right to wonder how these supports r built. It was a magnificent engineering feat to discover those methods & they r still the most common method, although these days most of the digging underwater to reach the bed rock is done with machines. Most people who admire great bridges never realize that their most amazing aspect is hidden under water. The center bridge support in the Bay Bridge in San Francisco is the biggest that I know of. Because the bridge is so long & elegant, the center support looks small. But if u saw it up close & could see how far below the bay is surface it descends, you'd really be amazed by its enormity.

Shallow water spans r sometimes built using pre-fabricated concrete supports that r floated to the desired location on barges & then placed using cranes. That is how the Chesapeake Bay bridge-tunnel was built. But the vast majority of long bridge spans over ocean inlets r built with the caisson method. There may have been some bridges built with the assistance of extra-terrestrials & anti-gravity methods, but these r unfamiliar to me.

This website offers some good photos and diagrams:
http://rtc.wa.gov/Studies/SR35/docs/t3ConstructionAssumptions.pdf

There are no bridges over oceans. Bridges that are very long require support. These towers are build by creating small round dams and removing the water. It can only be done in shallow water such as the bridge between the Florida Keys.

The military sometimes uses a floating bridge over rivers. That can not be permanent since boats can not get by.

floating bridges or cantilever

They use anti-gravity devices from UFOs.