Not necessarily. There are, as far as I know, only 5 ocean-to-ocean canals in use for freight traffic. Two of them do not have locks: Suez and Cape Cod. The Panama Canal has locks, of course, although that’s mainly because it’s not a sea-level canal. But if it were a sea-level canal it would likely need locks anyway. The Kiel Canal has locks for the reason you state. The White Sea-Baltic Canal is not a sea-level canal, so it also has locks.
Do you have a cite for this? Having lived within a kilometer of the Nagasaki bomb site, and having visited Hiroshima Peace Park with the Atomic Bomb Dome which was about 150 m horizontially from center of the blast, as well as having read extensively about the bombing, I’ve never read about a crater.
Trinity was conducted on a 100-foot tower and did create a crater.
I’ve read various numbers for the size of that crater, but there doesn’t seem to be any doubts that there was one. However, I haven’t seen any numbers that are anywhere as deep as you suggest for the Hiroshima bomb.