Foundation

Hi, I was hoping to build a house that was partly on land and water, but for some reason I cannot place the foundation further in the water. 

Its a big lot, (50x50), so there is plenty of land, 50% is under water, but its not for allowing placement of the foundation, and the land is level up to the sea.

Does anyone know why its happening and how to solve?

Cheers!

Here pic for details:

Why this happening?