The two main types of translation, word-for-word and thought-for-thought, give different answers.
As far as a thought-for-thought translation, such that the French speaker and the Latin speaker would have the same reaction to the utterance, I think the closest adage is:
Aide-toi, le ciel t'aidera.
God helps those who help themselves.
For a word-for-word translation, what you chose makes sense and would certainly be understood:
S'il n'y a pas de vent, ramez !
But there's a middle ground: a literal translation — often used to add colour to a text, to let the French reader feel they're learning something about Latin and about Roman culture — that also takes into account the conventions of adage in French.
Taking a look at one or two samples to confirm my suspicions, I would say that those conventions don't favour the imperative. They're often direct generalizations like Chaque chose vaut son prix, statements of value like Il vaut mieux plier que rompre, equivalences like Vouloir c'est pouvoir, or condition → consequence like Quand on n'a pas ce que l'on aime, il faut aimer ce que l'on a.
Given those observations, here are some some of the many options that might be more likely:
Quand il n'y a pas de vent, il faut ramer.
Sans vent, on rame.
Qui manque de vent doit ramer.
Edit: user168676, noting the tendency to omit articles in proverbs (perhaps because many predate the stage where articles are necessary?), suggests these as well, which I find also have very idiomatic transformations of the syntax:
Navire sans vent repart à la rame.
Navire sans vent continue à la rame.
Of course, since there isn't such a proverb in French, you still won't find examples if you search, but at least the genre will be recognized more easily.
At least, so say I. Native speakers are invited to correct me!