It's an unusual water ski, no one knows much about it is an example of the so-called comma fault--using a comma to connect two independent clauses. The comma is not a connector; it is a separator. The semicolon, however, can function as both a connector and a separator, and at the same time: It's an unusual water ski; no one knows much about it. If we use a comma, then we have to supply a connector--that is, a conjunction such as and: It's an unusual water ski, and no one knows much about it.
We're going to try it, then we'll decide if we'll keep it may seem less obviously a comma fault, because then seems to be performing the role of a conjunction. But then here is an adverb, modifying decide; it is not a conjunction. The comma should be a semicolon, or else it should be followed by and. Note, however, that We tried it, then decided is correct. It is not a compound sentence. It merely has a compound predicate, and the comma indicates the missing conjunction and--an odd role for a comma, which normally prevents conjunction, but the pause it indicates represents the missing word.
Exception: the comma to emphasize
The turns were easy, the wakes were difficult uses a comma instead of a semicolon to emphasize the contrast--in this case an antithetical contrast--between the two independent clauses. The comma is especially desirable if the second clause is made elliptical: The turns were easy, the wakes difficult. If we make it The turns were easy; the wakes were difficult, we lose some of the energy and pithiness of the original contrast.
We could quibble with this example and claim that it is not really an exception to the rule, but an elliptical sentence, with but understood after the comma: but the wakes were difficult. An understood word--that is, a missing word--is often more conspicuous in its absence than it would be if it were present, because the reader has to supply it.
In any case, the comma is occasionally desirable when the rule calls for a semicolon. She was not twenty, she was twenty-one uses the comma to emphasize the contrast between a negative statement and a parallel positive one. He was twenty, she was twenty-one uses the comma not to emphasize contrast but to emphasize the slightness of contrast. The turns were easy, the wakes were easy uses the comma to emphasize the absence of contrast. In all the examples of exceptions given here, the comma is not only justifiable but preferable.
If the independent clauses are considerably longer than they are in the examples above, if they do not balance so neatly, or if for any other reason the comma does not seem a sufficient signal to the reader that another independent clause is coming up, it is better to follow the original rule above and use the semicolon.
Exception: the comma to indicate a continuing series or to heighten parallels in a series.
JoJo couldn't ski because she had to study, Gracie couldn't ski because she was in a spelling bee, Jaybird couldn't ski because he was in a truck race. Only 50 percent of the team showed up. This is an acceptable use of commas. A seemingly desirable and is omitted after bee chiefly to indicate that the series could go on (like this week's post). Semicolons might not be quite as good, since the implication of an unfinished series wouldn't be as strong. At the same time, the commas make the parallel structure of each clause more evident. In a sense, a semicolon tells a reader to forget the grammar (if not the content) of what precedes, because a new independent clause is about to start. The comma makes it more likely that the reader will still be aware of the preceding grammar and will better appreciate the parallelism.