Out of curiosity, why do you think UW Nav is a good specialty? Do you get a lot of students who want it, or is there another reason? I'm kind of on the fence about that class because on one hand, instruction never hurts as long as the information is accurate, but on the other hand navigation seems like one of those things that you just need to practice.
Naturalist I do agree on, but as previously stated, if a marine biologist is teaching it and goes above & beyond PADI's requirements then it can be an extremely good class.
Emtim, The reason I think Nav is good is two-fold. With lower viz, getting students to trust the compass is key. Even good viz environments warrants it, but with lower viz (eg SoCal waters), I can usually demonstrate the benefit. I have often "wowed" the students in OW and AOW with how close we return to our starting point (boat/shore entry point). When explaining compass use and current reading, it is a natural roll into a class where they get to practice in a "staged" environment with someone to help out if they go astray - tough to practice on your own and find you missed the point .. by a lot potentially. The second reason is that you learn from someone how to read currents, natural navigation, sand lines, reference point usage, etc.
Yes, you can learn on your own, but reminding people that are new that they did not learn how to drive by just jumping behind the wheel with no parent/sibling/instructor probably found frustration. By getting hands-on help you probably felt more comfortable behind the wheel. Yes, it does entail some salesmanship, but putting the need for nav in land-based scenarios will hopefully demonstrate the value.
As for the Naturalist.. A marine biologist would add a TON of value.. the problem is getting that point across when discussing the class to people.. Again, you can sell it, but it entails more face to face discussion.. like saying WOW, did you see that XXX on that dive. His colors were amazing.. the way the green spots really made you think his eyes were elsewhere, etc... (example).
I have had a few students ask for Nav, and am teaching a group of 5 starting this week actually. Many of the people that discuss wanting Rescue, or a desire to learn, I highly encourage this class. It will help tremendously with the search part of Rescue.... and again, anything that build confidence in diving (eg compass use) is key.
I once had a group of divers on a boat where we all returned to the anchor line. We got in the boat, and one of them noticed 2 divers off in the distance.. the FIRST thing the diver said was... Gee, I guess they didn't know how to use their compass.. I couldn't help but smile... Sure, something else could have caused the oopsie, but I let that one sit....