Is there a submarine template in the creation mode?
NO! But don't worry, wasting 10th of hours trying to create one is the dumbest thing one can do.
They will shoot it into pieces within a minute anyway.
Your best bet is an aircraft with quite some firepower
(You don't need Missile Silos for these, but for another story mission to be completed)
You swim there, receive the message that you got what you are there for and then "sprint swim" (SHIFT!) back to the coast, jump into your aircraft and let lose on them. Trying to avoid being shot yourself isn't a bad idea.
Alternatively, lure them to the coast by shooting at them with an aircraft and then heading there. Jump out, TAKE your aircraft and shoot at them from the coast with something like this
...and why the hell are you running around in Silver Armor at this point in the story, still?