I wouldn't presume to dispute you, if you said water wasn't wet.

I wouldn't presume to dispute you, if you said water weren't wet.

As for the other two choices -- I personally don't see a reason to use the past tense. With verbs like "say", you'll often see sequencing of tenses in indirect speech -- if the verb of saying is past tense, the embedded verb is past tense too:

John said, "I'm posting to Hatrack."
John said he was posting to Hatrack.

But in this case, there is no requirement for sequencing of tenses since the description is supposed to be a generic statement, not one confined to some particular time:

John said, "Water isn't wet."
John said that water isn't wet.

If you said "John said that water wasn't wet," that would only make sense if John was talking about some specific water or some specific time frame, not making a general statement about all water in all times.

But, you know, I am not actually a grammar maven, so I could be wrong about this. My opinion is that of the three options you provide, the last one is best.

But how about avoiding the issue with, "I wouldn't presume to dispute you should you say that water isn't wet."

Just some ramblings...

Best of luck,

-K.

You can put the two clauses in either order:

I wouldn't presume to dispute you if you said that water isn't wet.

If you said that water isn't wet I wouldn't presume to dispute you.

But it does not bear on the question of what form of the verb should appear for "to be" in the if clause. But I think we're in agreement that it should be "isn't".

-K.