Forbes has a short article about anticipated advancements in the global submarine fleets. Technology is improving. More nations are starting to export submarines. Nations that have never had submarines before are putting them into service.
Perhaps most ominously a range of autonomous underwater vehicles with offensive capabilities are entering service. They range from torpedoes with artificial intelligence or mines that position themselves to a predetermined location, to an entirely new class of intercontinental range, nuclear powered, nuclear armed drones. In effect these include underwater cruise missiles. Since nations have seen fit to share cruise missiles with terrorist organizations, there is no reason not to expect that they will also share at least some of these weapons.