Ah, things are looking significantly better now, I assumed that the current turn was autumn (due to a critical reading comprehension fail), instead of that you just did autumn, and that he was going to move in and take smyrna. I didn't know you had a free build either, so your position is quite a bit better then I thought it was.
It might take a while but you should be able to take tunisia with your navies, but as long as you do it before one of the navies gets moved down from the north (2 years at the fastest), your 5 navies should be able to beat his 4.
As for tactical advice, its pretty tricky on how exactly to take tunisia. I would reccomend trying to take the thyrennian with naples, but as for his eastern navy you are on your own (possibly send a army down south to help defend?). Another option would be to move all your navies up one province, and try to move greece into the tyrennian. This has its own risks (namely that it could completely fail or that he *could* take rome), but most of the options for attacking are either slow or have some risks at this point.
After that it gets tricky, if you are lucky and move fast you might be able to get to the mid atlantic ocean (which would really open up your attack options), but if he gets it he can blockade you down south, quite possible stalemating it down there.
-----
It's hard to say what I would have done differently then you though, hindsight is always 20/20, and although your position looked strong gaining ground from it was actually very hard.
As it is though, you probably have 1 last chance to win before another navy or 2 come down from the north.
EDIT: I can see you easily taking the 17th province (tunisia), but if you don't manage to take the mid atlantic ocean, I am about 95% sure that the game will stalemate.