I would submit that sea trade routes are going to be a sufficiently broad and perennial part of the game that more than one civ can have a maritime UA. And Portugal probably won't have a monopoly anyways, because harbors will probably facilitate trade routes, making Carthage's UA at least as advantageous as Portugal's. I wouldn't hold any statement about Portugal being "the" naval trade civ to preclude anything.A problem is the thing Venice truly represents is Naval Trade, and I'm trying to steer away from that given Portugal has taken that and it's been called "the" naval trade civ.
Portugal's UA kicks in some extra money (doesn't even sound terribly "ulimate" IMO). There are other ways to go.
UA: Lifeblood of Venice. Military units have +10% combat strength near a trade route. Civilian units withdraw into the space of a nearby military unit when attacked.
UA: Masters of Coin. Great Merchants can be used twice before being expended. Upon researching Guilds, receive a free Great Merchant.
Of course, it's all very open-ended, with many ways to skin the cat (not that Venetians would ever skin a cat).