Frigates are indeed terribly unbalanced. 5-6 frigates and a privateer will let you conquer every coastal city in 1-2 turns practically without damage. The age of sail is just a big coastal city conquest and experienced players avoid to settle on the coast because of it. There is no land-based defense. Expensive cannons and forts will die easily to frigates and are immobile in comparison. Try it.[/quotes]
Experienced players who've pre-determined that they aren't going to build a navy are rather smart to then not build on the coast, yes. Like experienced players who've pre-determined they aren't going to build any mounted units are smart not to go out of their way to settle near horses. And how I, as an experienced cook, know not to stuff my hand into a soup pot full of boiling water.
I'd say a combinaton of nerfing frigates/privateers, add coastal fortress and reduce overall city damage to ranged attacks in general (with greater weakness to melee). Alternatively remove all naval bombardment damage to cities or something similarly radical.
This about realism.. well, the opium wars were not fought and won by frigates but by marines.
Aside from this being Tired Civ Argument #2, those marines (prior to the actual Marine unit) are represented in the game by...melee ships, which attack via boarding action. Those are your Privateers finishing the job in the age of sail.
I think something like giving anything with a bonus vs. cities a similar bonus vs. naval units or something that works vs. naval attack the way a bomb shelter works against nuclear (hell, do something like this for air raids too) wouldn't subtract terribly from the game, but they wouldn't add much either. There's no reason why the person who overlooks a rather simple precaution is entitled to immunity from that vector of attack.