Bethesda decided since it would have to delay the game again, decided to cut that content out, though it would've been cool to see a underwater world separate from the world we know and love.
Wrote: I solemnly believe that a future Fallout will include irradiated and mutated fish species, they might even add it as a mod in F4 in the future, I mean even Bethesda was planning on adding fish to the ocean as they had a speargun in the pre-release so yeah.