Only against the types this pokemon is weak to. Again, it's not really that broken if you consider that it's a mantine. While being immune to one electric attack will definitely help, there is still no way it'll be walling a pokemon with an electric attack, with or without the ability.Water Barrier - So does it defend against the first move of any type used against the Pokémon with Water Barrier, or does it defend against the first move of each type that the Pokémon with Water Barrier is weak against? The former makes it much like the Type Bits that I mentioned before in this thread (search for them if you'd like), but the latter would be extremely broken.
And if stealth rock is on the field, rock attacks will still do their regular damage as well. But if it really makes everyone feel better, I can say that it also halves attack and special attack....
Edit: Yeah, it is like those items, but only for it's weaknesses. Type bits are broken because they can work on any pokemon, though. This is only on Mantine, and really, in OU, I don't think it'll be broken. Anything with an electric attack will beat it anyway.