They'd have to be extremely bored to go much bigger than the Tsar Bomba, but then again I did hear mention of a Russian underwater drone capable of 100 MT, guess we can expect some Radioactive Tsunami's should it come to it!
If anything, the MIRV warheads deployed or in development would blanket large metro areas/small countries to the point where several smaller bombs would cause more Megadeaths (Look that one up) than one big boss bomb.
Also, my hometown makes me wish for nuclear winter! But no-one would waste a Tsar Bomba on such a mediocre target :p
A radioactive tsunami would probably be the least of our problems if that thing accidentally detonated in the ocean lol. I'm familiar with the term megadeaths and at least you are sort of safe in your hometown, you may not die instantly by an explosion but you will die anyways via radiation or the destruction of the whole world ecosystem.
It would be a 'in-the-moment' sort of deal certainly, obviously the fallout would do much more damage in the longer-term than the wave itself (Fukushima Tuna x100) nevermind the repercussions on something like that going off as a mere accident.
Safe would be somewhat subjective, considering my home is within the maximum radius of the blast front, if anything I would try to weasel my way to my cousin's house outside of town before the buttons are pressed...
Though as you said, global environmental collapse will turn every refuge into a grave, the Permian-Triassic Extinction will look like a cozy catastrophe by comparison.