Good questions.
First, I want to note that an aphorism is meant to be pungent, short, and puzzling. The point is to simplify a proverb down into a thought-provoking sentence, which will cause one to think about it more deeply exactly the way you are. (:
Second, let’s dive into its meaning.
Drinking salt water is ok, assuming the concentration is low, but maybe we are intended to think of someone who is drinking only sea water as opposed to broth, which is a death sentence. Drinking salt water is ok if you have the means to dilute with fresh water in alternation.
The immediate point is that salt water doesn’t
quench one’s thirst—not that it may kill you. Hope is the same way: when you are really thirsty—perhaps when stranded on a boat—it is really tempting to see the salty water as a viable solution, but the more you drink it the more it slowly
causes more of the issue needing to be solved. Hope, schopenhauer famously stated, is the confusion of the possibility of something with its probability: it to latch onto something in a manner where it is despite its probability being disproportionate thereto.
When we are in despair, we tend to see hope as a viable antidote—a solution to the problem—just like using salt water to quench one’s thirst; but, in reality, it is contributing to the problem. How so? The problem, the Stoic would say, is that the person is discontent with what is outside of their control; and the attachment thereto is causing their mind a disturbance—viz., from which the ‘problem’ arises in the first place needing to be solved—and procuring hope only temporarily alleviates the problem by consoling the person swiftly but making them, so to speak,
thirstier. Hope just makes a person more attached to what is not in their control, which adds fuel to that fire of discontent. The Stoic is going to say that it would be better—and a real solution—to detach from what is outside of one’s control and to work towards whatever one wishes with respect to what is within their control.
The important thing to note here is that the Stoics are talking about ‘hope’ as an irrational passion because it only arises when one is irrationally attached. They are not claiming that one cannot be wishful of the future—just that one needs to equally detached from what is outside of their control and they must be able to size up the probability of it occurring properly. Most instances of hope are not like this: they are, instead, irrational fits of emotion.
I think in some of what you mentioned, you are just using the term ‘hope’ in the sense of merely wishing or desiring; and that is not what is meant by ‘hope’ here. Hope, traditionally, is more than a mere wish—it is more than a mere desire—: it is much stronger than that.