There's probably thousands of scuba divers are trained in cold water diving to 300ft/100m depths. That is deep technical diving, but not crazy or stupid deep.
(Although it would probably be easier these days to use a pipe bomb and a COTS underwater tethered drone -- depending on what bits of physical evidence you were comfortable leaving behind you could just blow the drone up with it too -- the "hardest" part would probably be dodging the coast guard on the surface)
Besides motive, this article doesn't provide anything new. And that the US had at least motive is established fact since basically the day of the explosion.