Hi
I have got few papers which do not exactly explain the theory but gives an equation for calculating the secondary correction factor

" Loran is retarded when travelling over sea water as opposed to through the atmosphere. When transmitted, part of the electromagentic wave is in air and some in earths surface (sea, land). Sea water is not as good a electrical conductor as air, so the signals are slowed as they travel over seawater."
(I didnt understand the last line....How? and is sea water less conductive than air)

This secondary correction factor is calculated from Harris polynomials.

SF = -0.01142 + 0.00176d + 0.510483/d for d < 100 miles
   = -0.40758 + 0.00346776d + 24.0305/d for d > 100 miles

SF is the additional time to be added to the time calculated from d/c

National bureau of standards has  a set of generalized curves for incremental propagation time for propagation paths as a function of distance.
These curves are calculated from the above equations.

This is the best and the closest thing I have found for my question. I dont know if we can find this on internet.

I found this in the university library.

I hope this is helpful and if possible can someone explain
"Sea water is not as good a electrical conductor as air, so the signals are slowed as they travel over seawater."