User blog comment:Kogath/Monster Hunter X Discussion/@comment-4090136-20150927052759/@comment-25119729-20150927161244

Also note the fact that in the Lagaicrus fight you can see the underwater part of the area the Lagiacrus is fought in, so the pure underwater areas simply are no longer accessable I think. Which would mean it is like FU, there are underwater areas, but you simply cannot enter them. Far better than P3rd's approach to that matter.