My bad, thought you were talking about US subs.
My argument still holds. I know how those are designed and some safeguards designed to keep them from having major problems with leakage if they are sunk. You don't because it is classified.
Now, it is possible that the other countries' subs aren't designed as safely as ours. But they still are not an issue. Water is a huge shield to radiation. It is also a moderator, which means it slows down and diverts the worst radiation given off by nuclear reactions.
They do not rust and deteriorate that fast. It would take a long time for the ocean to actually eat into the reactor itself. There is a lot of containment there.
You are fearing something that really has no reason to be feared.