I thought so to, but looking at the numbers that doesn't seem to be correct, f.e. there a lot of 0-values in the list. And those provinces do have access to the sea.
Or Hampshire has 27, but province 27 is Dorset
Kaisereia has 1293. but there is no province with such a high number
So I rather think that the value doesn't have any effect at all, probably some from the orginial development team of the game that never made it into the final game.
Just like 'Muslim_duchy', 'Muslim_kingdom' and 'Papal_duchy' in the same file