Gastrodon
- Storm Drain is a nice Ability
- Excellent typing gives it one weakness
- Very high HP
- Solid Special Attack
- Learns moves like Surf, Ice Beam, and Sludge Bomb
- Very slow and average defenses
- Shallow movepool
- Best special Ground move until the post-game is Mud Bomb
Floatzel
- Superb Attack and Speed
- Evolves earlier than Gastrodon
- Learns moves like Waterfall, Strength, Brick Break, and Aqua Jet
- Frailer than Gastrodon
Go with Floatzel. Offense is everything in-game, and it's more offensive than Gastrodon.
Floatzel @ Mystic Water
Ability: Swift Swim
- Waterfall
- Brick Break / Ice Beam / Ice Punch
- Strength / Ice Beam / Ice Punch
- Aqua Jet / Crunch
If for some reason you decide to use Gastrodon:
Gastrodon @ Mystic Water / Soft Sand
Ability: Storm Drain
- Surf / Muddy Water
- Mud Bomb / Earth Power / Earthquake
- Ice Beam
- Sludge Bomb / Recover
Hope I helped!