NavList:

I'm going to second Frank's recommendation of a Davis Mark 3. They are in fact accurate enough for serious navigation. They are light weight, rugged, simple, reliable, just about impervious to salt water corrosion, and inexpensive, but they have still more attributes that are rarely mentioned:

They are perfect for those days when the spray is flying and you don't want to expose your expensive metal sextant to a salt water dousing. They are just the thing for the ditch bag, or for a backup to your fancier sextant, or to use to teach curious crew members about cel-nav, or if your crew member has learned the basics for them to use to also take sights alongside you.

If you start with a Davis Mark 3 you will wind up keeping it forever for these reasons even if you upgrade later on to a fancier metal sextant, so you may as well start out with one to begin with.

With the money you save you can purchase almanacs, plotting instruments, a time piece and a backup, (you can't get the time form the GPS when the GPS is out,) sight reduction tables with worksheets to match, instruction books, and various other bits of kit which you will find that you need for cel-nav.

When looking at metal sextants they should not be polished. If you come across one that is polished brass or bronze so that it gleams in the antique store window – skip it. So no sextant worth having aboard is going to match your bronze port lights or antique lamps.

In my personal experience the more expensive and fancier plastic sextants are not as good as the humble Mark 3 – but they cost a whole lot more.