Harbours actually give additional Tourism on sea TR completion, btw. You can stack that with the Caravansary buff IIIRC. And Harbours are more generally useful/built more often than Caravansaries, at least in my games. I often find that defending sea routes is actually easier, more or less. And while you absolutely should have a few land routes to pass over villages, the bonuses don't stack and so you don't need exclusively land to benefit from that. I don't feel the need to go and buff International TRs more, sea or land. They're quite balanced and sea TRs have the inherent value of reaching far-away lucrative targets that land routes don't.