If you're going to do something as lame as give an immunity ability to cover a 4x weak, Fire/Flying + Rock Immunity is way more awesome.
Fire Flying + Rock Immunity:
Weak: Water, Electric
Immune: Ground, Rock
Resists: 4x Bug, 4x Grass, Fighting, Fire, Steel
Only 2 weaks predominantly special? Sounds like Skarmory . . . no, actually, this looks arguably better than skarmory.
Gyarados Waterfall, TTar's Crunch, and Outrage. Outside of those, you resist/immune almost every other prominent physical attack in the game. Throw in Skarmory like Defensive stats, Lava Plume/w-o-w and Roost (almost guranteed for fire/flying) and you have a ridiculous physical wall.
OU Physical Sweepers out-right walled:
-Scizor
-Tyranitar (ever try breaking through Gliscor with Crunch? It doesn't work)
-Jirachi
-Metagross
-Lucario
-Infernape
-Breloom
-Machamp
-Aerodactyle
-Weavile
-Mamoswine
-Heracross
Remaining OU Physical Sweepers:
-Gyarados
-Salamence
-Flygon
-Kingdra
-Electivar
Out of these, Electivar sucks, and Salamence/Flygon have to resort to Outrage if they even want a chance (unless they carry Draco Meteor). Gyardos and Kingdra are the only physical threats that it can't at least check, and even they have to be afraid of burn.
On top of that, you have immunity to SR, Spikes and Toxic Spikes (and maybe even to sand if you construe sand as "rock" damage, sandstorm being rock type). IE. this thing is extremely difficult to wear down.
Basically, if you're going to give out an immunity to a 4x weak, there are better options than fire/rock + water immunity . . .