Underwater foundations can be constructed in several ways:

1.  Precast structures which are lowered into place and anchored to the floor.

2.  Inflatable formwork.  The shape of the foundation is manufactured into a material "shutter" into which the concrete is pumped.  The material is placed on the formation and the concrete is pumped in.

3.  By far the most common method is to cast the foundation using a tremmie pipe.  The pipe is lowed down to the formation and concrete is pumped in from the surface.  You need divers to manipulate the bottom of the pipe into the correct position while pumping.  The key is to keep the bottom of the pipe buried in the wet concrete to prevent the cement paste washing out.  The divers move the pipe around until the correct shape is formed.  The concrete mix is overedosed with cement to account for some of the cement leeching out into the surrounding water.  

The water in the mix does the reacting with the cement.  It is usual to assume that the top 75mm thickness of the concrete is useless and that it has been contaminated by the surrounding water against which it has direct contact.  The water will not leach down into the concrete provided that you use a good controlled placing technique.  You may wish to add micro silica fume to the mix which will make the mix react quicker and make it much more cohesive (both of which mimimise the risk of water contamination in the pour.

Andy Machon