In the comments to the post below, Crimson Rambler raises the excellent question of the correct past tense of the verb "to tread." Simply put, is it correct to say "we treaded water," or is "we trod water" better? Well, after much running around in the warrens, and some rather focused poking of the Googlemonster, we arrived at the conclusion that both, in fact, are just fine.
First of all, the subject of the past tense of "to tread" was broached not once, not twice, but at least thrice by a certain Professor Henry Bosley Woolf of Louisiana State University, a specialist in Old and Middle English, and one-time Editor-in-Chief of Webster's Collegiate Dictionary. In his three articles, written in 1946, 1953, and 1954, Professor Woolf arrived at the conclusion that, while "trod" had the weight of history and the dictionaries behind it, "treaded" had certainly gained some currency (full citations are given below). As Professor Woolf noted, strong verbs (those verbs that indicate the past tense by changing their root vowel, e.g. "run" to "ran" and indeed "tread" to "trod") can become weak verbs (verbs that indicate the past tense by adding a dental suffix, e.g. "form" to "formed") over the passage of time. In fact, according to this BBC webpage, the number of strong verbs in the English language has fallen by nearly by nearly two-thirds in the past millenium or so, despite the fact that English vocabularly has grown immensely. Therefore, it would be entirely reasonable to suppose that "treaded" might at some point completely replace "trod" as the past tense of the verb "to tread." Prof. Woolf did mention the possibility that "tread" was the correct past tense, but could find not examples of it in use.
Seeking a definitive answer as to whether "treaded" could yet be considered correct, we consulted the Oxford English Dictionary, to see what a more recent source might have to say on the matter. And, sure enough, under "to tread" we found "treaded" listed as an acceptable weak form of the past tense... but only, only, in the phrase "treaded water!" This would seem to indicate that "treaded" is analogous to "flied," the weak past tense of "to fly," which is only acceptable in the context of a batter in baseball hitting the ball such that it is caught by an outfielder before it hits the ground (UPDATE!: Not so! See Crimson Rambler's comment below!).
So there you have it! Both "trod water" and "treaded water" are acceptable phrases, although the latter is much more recent, and is still coming into common use as a past-tense form in general.
However, we were not finished! To try to get some sort of idea of how far "to tread" has travelled along the path to weak-verbdom, we turned the Googlemonster loose on the problem. Here are the number of results that he turned up for the weak and strong past tenses of "to tread" when used in a number of common phrases:
|Infinitive Phrase||Strong Past Tense ("Trod")|
Weak Past Tense ("Treaded")
|"to tread softly"||3,190 (70.4%)||1,340 (29.6%)|
|"to tread carefully"||5,010 (59.4%)||3,430 (40.6%)|
|"to tread lightly"||4,740 (50.1%)||4,340 (49.9%)|
|"to tread firmly"||946 (100.0%)||0* (0.0%)|
|"to tread the boards"||15,500 (97.4%)||421 (2.6%)|
|"to tread water"||6,820 (25.3%)||20,100 (74.7%)|
*Actually, there was one result for this, but it was quite clearly a typo for "threaded firmly."
We can see from this that, while the weak form now dominates in the case of the phrase "to tread water," and has a significant presence in a couple of other phrases, we still tend to use the strong form of the past tense.
Prof. Woolf's articles (you will need a JSTOR account for the two linked articles):
American Notes & Queries, Volume V (1946), pp. 168-169.
"Trod Water?" American Speech, Vol. 28, No. 3 (Oct., 1953), pp. 234-235.
"'Trod Water' Again." American Speech, Vol. 29, No. 4 (Dec., 1954), p. 294