Excellent point, Lowell!
I'm now wondering if the intent might also be to cause widespread damage tens (or maybe even hundreds???) of miles up and down the coastline from a city? If you can cause total devastating damage along a 200 mile stretch of coastline, that might very well be worth the expenditure of eight nukes. Woe be to any military installation, important infrastructure, etc., that happen to lie along that stretch of the coast.
I just don't know how far a big tidal wave can extend up and down a coastline. Couldn't find any mention on Google about, say, some prehistoric giant tidal wave that wiped out a thousand miles of coastline in twenty minutes. Then the city angle is just a ruse and this becomes more about a mighty body shot that brings a giant to his knees, especially if it's a simultaneous attack along both coasts and the gulf.
https://www.lpi.usra.edu/science/kring/Chicxulub/regional-effects/
However, humanity does not have the ability to generate the blast energy required to create a Chicxulub-level tsunami. Large natural tsunamis that we see with some regularity are also beyond man’s ability to reproduce. That said, significant tsunami events can be generated on a local scale with nuclear weapons and those can do significant damage.
It is at least theoretically a viable tactic and has been looked at before: https://en.wikipedia.org/wiki/Tsunami_bomb