80 HP means Mantine won’t be donked easily, although the Lighting weakness puts Mantine in danger of Pachirisu donk. Fighting resistance is nice, but it doesn’t change the number of hits Donphan needs to KO Mantine. 1 Retreat Cost isn’t much, but it’s poor on a starter.
“Group Swim”, for one Water, allows you to search for any Water-type Pokémon in your deck to move to your hand. Since this includes Evolutions, it’s a valuable move in a Water player’s arsenal.
“Aqua Slash”, for two Water, will do 40 but prevent Mantine from attacking the following turn. I can’t think of a particularly good reason to use it, but perhaps you can pick up an easy prize on Cleffas.
Mantine is probably a more consistent and definitely a safer starter than a Baby in Water-type decks. I find it to be more useful for finding Evolution cards than Basics, but you have the option if you wish to. In decks that use more than one type, it’s probably better to not run Mantine.
Obviously, don’t run Mantine if your main Pokémon aren’t Water. Seriously.
Mantine gets a 2/5.