Jatoba (aka Brazilian Cherry) is an absolutely beautiful wood, and is very dense. As a matter of fact, I'm typing this message on my custom Jatoba wood desk
I haven't used Jatoba on my boat. I'm using Phillipine Mahogany, African Mahogany, and teak (just for the hand rails)....but...
Jatoba is very resistant to rot and insects. It is susceptible to marine borers (like Shipworms, Teredoes, or Gribbles). Since you're not building pilings out of the wood, you're going to be OK here

Also, marine borers are saltwater, so if you're a freshie, you're OK. Lastly, any surface barrier will protect the wood from a marine borer...so likewise, you're fine there.
OK, coverage...
Use a clear varnish with or without UV inhibitors (Jatoba turns a beautiful dark red when it is exposed to sunlight, if it's not dark red already; and this change happens within days outdoors.) I used Z Spar Flagship varnish with excellent results on my exterior teak and mahogany, but never tried it on Jatoba.
If you want to use some for interior work, go with varnish or polyurethane. I use Minwax Wipe-on Polyurethane (Gloss) exclusively in my interior. Jatoba will take about 6-8 coats for a nice glossy, but not the super-glossy varnished look. Wipe-on poly, unlike its brush-on cousin, is very easy to apply.
I would not recommend Cetol on Jatoba without doing some test areas (and then only Cetol Light) only because Cetol has pigments and I believe it is a federal crime to cover up Jatoba.