Speaking of fish/mono U.....
I tried this at the last waterbury tournament. Didn't do so hot, but it was interesting, and old man was a brute. Not having man-lands hurt, but I really wanted to pack back to basics. Oddly this thing ate a food-chains deck alive, which was highly unexpected. I thought piledriver would own me, but the sword of fire and ice turned out to be a highly effective goblin killer.
4 Spiketail Hatching
4 Flying Men
3 Old man of the sea
3 Suq'Ata firewalker
(14)
4 Curiosity
3 mana Drain
4 Force of will
4 Standstill
2 Cunning Wish
(17)
1 Ancestral Recall
1 Timewalk
3 Back to Basics
2 Sword of fire and ice
(7)
1 Black Lotus
1 Sol ring
1 mox Sapphire
4 Wasteland
1 Stripmine
6 Fetchland (3 polluted Delta/3 flooded strand)
8 Islands
(22)
SB:
2 Gilded Drake
1 Old Man
1 Suq'Ata
3 Sigil of sleep
4 Null rod
1 Back to Basics
and some other stuff I can't remember because I never sided it in. Probably a stifle, a manadrain, and a chain of vapors. They were wish targets, but I don't think I ever had a chance to wish in the entire tournament.
4cc killed me by cycling through standstill a few times, but it wasn't a total beatdown. Some sort of slavery build sans slavers was a draw; gilded drake stole a platinum angel there. I also used it to steal an exalted later.
I came expecting workshop decks, just like everyone else, and got owned because of misreading the meta.
A U/R oldschool counter-burn deck slaughtered me. My counter was too light and my creatures too wimpy to handle it. I also got clobbered by white weenie...his crusade made his critters too big for my old men to steal
If this had been U/R fish I would have won all the games I lost, honestly. I missed the grim lavamancers terribly.
That's what I get for going all creative the night before the tourney and not play testing enough

Edited to add: Forgot the point of my post:
Mono U seems to work because of superheavy counter and hosers like BtB and Control magic.
U/R Fish seems to work because of laying small but numerous threats down fast and hard and drawing massive amounts of cards to compensate for the lack of strong counter.
Fish/Mono U removes the heavy counter present in straight mono U and ditches the null rod hosers, and in the process adds more counter to U/R fish and takes out the hard hitting critters like lavamancers.
It seems like it would be a good combo, but it takes the weaker aspects of both decks and tries to mash them together.
I'm not saying it couldn't be done, just not quite like this.