Wasn't sure where to post this but here goes...
Where: Serpent Lake, Zangarmarsh
Mobs: Mudfin Frenzy
This is a very quiet place since most ppl dont even think about underwater grinding. Anyway the Frenzy's are normally pulled in groups of 4 and if you're a mage put up Molten Armor, pop a waterbreathing pot and you can pull almost unlimited sized groups. I was swimming around pulling 3-4 groups at once and one Blast Wave or 2-3 Arcane Explsions take care of them since they only have about 1.2k HP.
They drop mostly Grime-encrusted Scale's which can be sold at vendors for 1.6g per stack, Ripped Fin's that are sold at 1.8 per stack, the grey item that are sold for 2-3g apiece and last but not least they have a high droprate of Unidentified Plant Parts that is used for Cenarion Circle rep.
I get about 60-70g per hour not to mention selling the plant parts at AH for about 6g for a 20 stack.
I hope this will be helpfull since alot of ppl dont know about this spot ^^