This locution has many versions, but just this one is misspelled. It should be "never say of this water I will not drink"; if you are not missing dialog scripts or quotation marks.
In my country (Colombia). It refers to a person who annoys or dislikes another, or he may be disgusted or annoyed to do, consume, or manipulate something and for such facts he says figuratively (He will never do it or that he will never eat it).
It is said when we affirm a fact or action and make it clear that we will not be part of it. And, after time, we see ourselves in the need to be part of such fact or action. Examples : I would never fall in love with a person like that. . . and after a while that person is our boyfriend (a) and sometimes husband ( a) . Similar to this expression is "a talker falls sooner than a lame one."
It means you should never insure something as conditions or situation change by changing our position on this issue
it is something like O.E. : never have to say never, because at the least thoughtful moment "that" can happen to us. Another maxim to explain this sentence would be "By mouth dies the fish". Because if you're talking, you're going to have to swallow your words at a certain time. Another saying that might serve to explain the case a little more is that of "Never spit up". . . because the spit could fall back into your mouth.