The Water Weird never showed up in any of my Rules Cyclopedia (bless it's holy name) days, though apparently it was in the B7 adventure module. I'm not sure it's a monster that I ever would have come up with, or used at the time either.
In my second game of AD&D 2nd Edition I rolled up a low level cleric to join a party of adventurers who were all at least 1 level higher than I was. I don't remember what the adventure was about, but I do remember that we had to cross a lake in basically a big row boat. We were attacked by a water weird right in the middle of the lake, and one by one it knocked my companions off the boat and into the lake. I was the last person in the boat. I can't claim that my character was the last character standing, as he was in fact cowering at the back of the boat, holding my light mace and holy symbol.
It was looking grim. The armored party members were desperately trying to shed their armor before they became permanent lake inhabitants, the unarmored ones were trying to catch up with the boat that the weird had pushed away from them, and my initiative came up.
Everyone looked at me sadly, knowing the fate that was to befall their newest, youngest, least experienced player and character. The DM sat smiling behind his screen. I gulped, and looked once more at my bare character sheet. Do I have anything in my inventory that might help? Non-magic items, and not many of them. What about spells? Cure Light Wounds, and Purify Food and Water. I re-read the description of Purify Food and Water:
When cast, this spell makes spoiled, rotten, poisonous, or otherwise contaminated food and water pure and suitable for eating and drinking. Up to 1 cubic foot of food and drink per level can be thus made suitable for consumption. This spell does not prevent subsequent natural decay or spoilage. Unholy water and similar food and drink of significance is spoiled by purify food and drink, but the spell has no effect on creatures of any type nor upon magical potions.I had nothing else. I meekly said "I cast Purify Food and Water."
The DM described how the weird arched over the boat, moving toward me, and drenched me as it fell dead onto me. My fellow player's jaws dropped. The DM continued to grin. Slowly I went from a cringe to a grin and yelled out "YES!" There may even have been a fist pump. A chorus of "Yeah!" and "Whooo!" arose from my fellow players, and everyone got back in the boat.
The DM showed me his Mosterous Manual when I asked about the spell having such an impact after the game. Once the euphoria wore off I had been left to wonder if the DM had just given us a monster we couldn't handle, and used me to pull his fat out of the fryer, or if it had really been supported. I was even more thrilled to find out that I'd actually pulled it off.
That was the first, and last time I've encountered a water weird. I've never used one in any of my games, mostly because all of my 2nd ed players (once I started DMing) had been part of that group, and they'd all seen the trick, and by 3rd edition I had other monsters that I was playing with.
I don't remember if the DM had seen my spell selection or not, or if he was running a home brew adventure, but I'll always remember that encounter, and how much it rocked!