A lot of creative thinking is going on here.
I personally am not going for the sources in the sea theory. My ideas run
along the line that the constant movement of the water disperses the
magical energies. This bordering effect of moving water also creates the
layout of provinces as underground waterways constrain the area from which
a source can collect the Mebhaighl.
But if you want to expand into the Ocean you can ofcourse. One thing you
have to think about of course is the edge of the continental shelf.
You have coastal waters, the area of the shelf and the deep ocean.
Most of the marine life is concentrated in the top layer of the water so
the coastal and shelf areas would be the first to be explo(it/r)ed.

Jan Arnoldus