It has a whopping 140 sp def and 85 hp. It has haze and roost, along with the ability water absorb. It has mirror coat too for some extra damage. It's an amazing sp wall but is only in ru. Not very good. So why is it ranked here?
Mantine is very slow and passive, and it's limited to particular archetypes, like defensive and balanced teams.
...both can help with bulky Water-types, which Mantine stalemates with at best.
Mantine is too slow and defensive to be worth a whole lot, and it can't defend very well against bulky Water types. It also has a crippling weakness to Electric.
Source
Hope I helped!
The reason why mantine is RU is likely because of its 4x electric weakness. Thunder Bolt is one of the most common special attacks in the game, and can be paired with Ice Beam for the infamous Bolt+Beam coverage combination. This is very commonly seen in most special attackers that have access to it, of which there are many. It also lacks good abilities, since they are all situational. It's defense stat is rather low, and the fact that it is slow does not help either. In addition, it is pretty passive, so it gets shut down completely by taunt if not using Scald. Stealth Rock also chips it down.