Storm Drain - If a single-target Water-type move is used, it will be forced to strike the Pokémon with this Ability, regardless of the Pokémon originally selected as the target and regardless of the move's accuracy. Moves which target multiple Pokémon cannot be redirected. In Generation V and beyond, the move will raise the Special Attack of the Pokémon and deal no damage to it.
Muddy Water inflicts damage and has a 30% chance of lowering the target's accuracy by one stage. Muddy Water targets all adjacent opponents.
Since Muddy Water targets multiple opponents, it can't be redirected by Storm Drain. Hence, it would hit both the opponents, but it would activate the SpA boost.
If you can, could you also say the results if one of the opponents Pokémon has Storm Drain, or if the Ally of the Pokémon who used Muddy Water does?
Muddy Water can't be redirected, as per above. But in case of other moves, they can be redirected to both ally and opponent, regardless of the original target.
Hope this helps!