I always viewed seafloor missiles as kind of idea that looks promising at first glance, but didn't hold water (despite being water-tight; sorry for the pun) after close examination. Too many contradictions and weak points.
Firstly, such weapon would need to be fully automated. I.e. it would have no two-man rule, no human input at all during launch. I.e. the ability to launch strategic weapon would be completely given to machine - and if machine would made a mistake or suffer a malfunction, there would be no one to stop it from launching an attack. Sure, it's almost improbable... but you never could be completely sure that no fool would accidentally put a wrong tape into capsule computer.
Secondly, such weapon would be extremely dependent on communication channels. Communication with submerged submarines are still a big problem. And seafloor missile capsule would not even be able to rise for communication. Basically the ability to issue a launch order would depend on just a few low-frequency systems. And if the opposing side disable them by sudden strike or find the way to hamper with their transmissions (which is hard, but not impossible), all seafloor missiles would instantly became useless.
Thirdly... the opposing side, yes. While finding seafloor capsules could be problematic, it clearly would not be impossible. What would be impossible is to prevent the opponent from tampering with capsules as soon as it would found one. Security devices and booby traps could be circumvented. So at very least the opponent would get a working sample of strategic missile, nuclear warheads, underwater communication receiver, ect. At worst - the opponent may gain enough knowledge to efficiently seek for other seafloor capsules (and either plant bombs/jammers on them, or just knock them out).
Also, I'm not sure what would prevent someone from claiming such seafloor capsule as salvage - after all, it's in international waters and owner did not admit where exactly it was placed. So...
