NavList:

Thank you, Greg and Michael, for all your hints, very helpful. Are certain aluminium sextant models known for corrosion problems and should be avoided?

Martin, when you say that the Freibergers have brilliant optics, does this include the current 8x30 Porro prism telescope?

If I end up with a Freiberger, I'll keep in mind what you say about salt water ingress. I would imagine that salt water getting into the drum-enclosed micrometer mechanism might cause problems. Maybe rinse with fresh water or liquid oil if this happens? Maybe, when the Freiberger was designed, rough handling in the merchant marine was more of a concern than salt water spray on a yacht, thus the enclosed arm and micrometer? Freibergers, though, seem to be offered much cheaper second-hand than equivalent C&Ps. 

I have thought about boxes. C&P offer what they call "rough cases" (waterproof, shockproof, floatable) with some of their new sextants. Looks like an attractive solution. If I find a (used) sextant that I like and take to sea, I might try getting something like that and adapting the padding (which is the challenge with generic "rough cases" bought separately).

Martin