I mean, if you're going down that route, you can just tie concrete blocks to someone and shove them into the water. It's cheap and tested thoroughly by the mafia
I believe the point of OP’s “thought” Is that death-by-implosion is basically instant (30 milliseconds) so it could be considered more humane than other methods.
They’d have to descend in it for hours wondering when it would blow. That wouldn’t be humane at all - it’s like standing in front of a firing squad for 1-2 hours wondering when they’re going to shoot.