That's a good question.
First off, there's no way it's 16 days via boat to Riverfall from Zeltiva. I know the chart you're looking at; it just hasn't been corrected to reflect proper trip time.
According to the
Distance Chart, Zeltiva is 2,750 miles from Riverfall straight as the crow flies. By sea, it's easily three times that, so 8,250 miles, more or less. The best boating technology available in Mizahar is at about the level of a Columbus-era triple-mast caravel, which can't go any faster than 100 miles a day. Even given a favorable breeze, you're looking at about 80 days' worth of travel.
As for fares, there aren't any passenger routes in Mizahar, except maybe from Nyka to Mura, or Sahova to Sunberth. The only people making boats that can travel extended distances are the Zeltivans, and the only other people IC who have any are the Suvan pirates, who probably aren't doing passenger runs. The Zeltivans run purely cargo/trading routes.
Which isn't to say that they won't sell a berth or two to an intrepid traveler. Since they're making their money elsewhere, a berth would be a pittance -- maybe ten mizas. It'd be bring-your-own provisions, but a run to Riverfall would probably stop at Yahebah, Ahnatep, and Abura at the very least, so you'd have plenty of chances to re-load. It's slow, but it's probably the safest way to travel in Mizahar if you're going anywhere other than the roads that the Syliran Knights patrol. (You've reminded me that I need to finish the map of trade routes.)
Hope that helps!