Some facts and some information.
Waves propagate through a medium, they do not displace the medium.
Tsunamis need a "reflecting" source to focus their energy.
Energy in a wave dissipates at the square or cube (fractal effects, the geometry matters) as the wave expands.
Water is one of the best radiation shields (rivalling lead in efficacy) available.
Cobalt60 is 3 times heavier than water. It sinks.
Cobalt60 has a half-life of ~5 years and decays to a stable isotope of Nickel.
Tests at Bikini Atoll had the ships ridiculously close to one another in very shallow water.
alpha and beta radiation are very easily protected against.
gamma and xray radiation are stopped by dense metal...or water.
Half-lives: that which burns bright, burns fast. Granite has one of the longest half-lives known. Yet, we don't put our pots on top of granite counters and watch our food cook.
It would be more effective to place a nuclear mine in a harbor than to try to use this against a nation's waterfront. Heck, it'd be simpler for Iran to stick a nuke in a container and hide it on a container ship and sail it in.
All of which is mostly irrelevant.
The amount of energy liberated in a 100 megaton subsurface detonation would send an immense cloud of radioactive debris into the Stratosphere, The cobalt 60 would vaporize and go right along with it. while you are correct in that immediate detonation site effects would not be as severe as people probably imagine due to most of the energy liberated going into the creation of a new, self illuminated artificial harbor, the long term effects over the rest of the planet and downwind of the detonation would be catastrophic.