If your only objective is to take the class and then immediately write a certification exam for, say, Sail Canada or the Canadian Power Squadron, then no...you do not need a sextant.
But celestial navigation is a perishable skill. If you don't immediately start practicing what you learned in class — on the ocean if you are near one; with an artificial horizon, if you are inland — then four weeks after class ends you will have forgotten how to take sights and work up a fix.
If the idea of losing all you just learned disturbs you, then yes: you will need a sextant.
You basically get what you pay for. But you can buy a sextant for less than $100 that will let you learn...and which people have used to navigate across oceans. You will give up some accuracy in your fixes, but you will still be accurate enough to sail from Boston to Bermuda, and FIND Bermuda.
More than one sailor, in the pre-GPS era and without much navigational skill, has completely missed Bermuda by being just over the horizon. But if you are practiced, you can find Bermuda just fine with a $100 sextant.
Here are your choices. CA.Binnacle.com typically offers free shipping on items over $99.
Davis Mark 3 Sextant: $70 CDN. It looks and feels cheap - but it will work for you.
Davis Mark 15 Sextant: $235 CDN. This is also a plastic sextant, but has a drum micrometer and a better set of sun-filters, like professional-grade sexants have. It is more fun to use than a Mark 3.
The weakness of any plastic sextant is that if it is a sunny day, your sextant will change shape as it warms up from one sight to the next. And ironically, since it costs more money to buy, a Mark 15, with its black color, will absorb radiation and warm up more quickly than the white Davis Mark 3.
This warming/deforming can introduce some moderately serious errors in your fix. There are ways to compensate, but it is a bit of a skilled task to make the best possible use of a plastic sextant. An experienced navigator will be able to get better results from a Davis sextant than a novice. But still, if you don't have much money to spend, a plastic sextant is a good bit better than no sextant at all.
For reasons that you will learn more about during class, the Davis Mark 25 sextant is not recommended.
...but also quite a bit more money is the Astra IIIb Sextant: $900 CDN This is an aluminum sextant that will serve you for the rest of your life, and will give you terrific navigational accuracy while doing so.
When you buy one of these, be sure to get it with the "traditional split mirror".
If you take a shot with an Astra that gives a significantly different position than the GPS on your phone would suggest, it is less likely to be that your sextant is acting up, and more likely that you have made some sort of mistake (or perhaps that the Russians are spoofing your GPS signal).
On the other hand you are using one of the Davis sextants, it can be a painstaking process to sort out whether an error is due to the plastic warming up and deforming, or to user error.
While a Tamaya Jupiter Sextant will not give you significantly greater accuracy than an Astra, you can feel that it has been carefully constructed with attention to details as a precision device. It still has the light weight of an aluminum body, but combines that with a bronze arc. This will give the maximum possible accuracy in sights, in an instrument will last not only for the rest of your life, but that of your children and grandchildren. Of course, at $1,700 US, it will be more than twice as expensive as an Astra. But just holding a Tamaya can brighten a cloudy day, and taking your first sight with one will add a couple of years to your life.
This sextant will give terrific accuracy for decades as long as it is never dropped nor picked up by the arm instead of the frame. Over a century ago, an influential navigation instructor said, "Never loan a man your horse, your gun...or your sextant." These are words to live by today.
A Cassens & Plath Standard Model sextant comes in at exactly the same price point as the Tamaya, but has a solid brass body. When taking a shot from the deck of a 45 foot yacht in high waves, it can take several minutes to get the sights you want. Even with an aluminum sextant, you can find that your arm starts to tremble from holding the instrument. A brass instrument, because it is heavier, is worse.
If you pump iron, and will be taking your sights from the deck of a supertanker, go ahead and get a brass sextant. But if you are a small boat sailor, the Tamaya is a better choice.
The Tamayas come standard with a moulded plastic case. If you get an Astra or a Cassens & Path with its wooden case, you should consider getting an all-weather case to go with it.
Actually, since a separate molded plastic case adds $125US to the cost of a C & P sextant, a Tamaya is actually more economical than a Cassens and Plath.
Something to Avoid
The folks at NASA really ARE rocket scientists...but that doesn't mean they actually recognize the difference between a real sextant and one that has been designed for display on a coffee table.
This page illustrates a "sextant" with clear glass for sun filters, and a telescope with a pinhole in place of the objective lens. It is unlikely that you could even see the sun with such an instrument, and if you DID somehow see the sun with it, you would end up with permanent damage to your retina. This instrument even lacks a handle to hold it by while attempting this sight that would ruin your vision.
Unless you are already skilled at recognizing quality navigational instruments, stay away from eBay. You risk getting something that LOOKS like a sextant, but was never created to be USED as a sextant. There are a ton of "coffee-table sextants" floating about in the world, designed to add a nautical flavor to somebody's den, as well as actual Tamayas that have broken micrometer drums. None of these can be used to take an accurate sight. You would be better off with a brand new $70 Davis Mark 3. It may be inexpensive, but what it DOES do, it does well.