Assad knew what would happen so I assume he has access to 19 subs and

[The Straight Story]

10. If you think Iran is going to risk their subs I would disagree

The us has roughly 72 subs. And if you want to count allies as you are doing with Iran then we have a lot more to add.

We have one of the best sub hunting fleets in the world, and Iran cannot afford losses of them over Syria. Their subs would be patrolling the gulf of Oman and perisian gulf. They would take time to get to the Mediterraen sea (and would have to pass some of our ships and our allies' ships to get there, not an easy route there).

We could place a few subs strategically to intercept them leaving their main areas and most likely have already done so.