Samurott (Obviously)
Samurott is a great starter, and helps a bunch against many gyms. It has a varied movepool, and can be physical, special, or mixed.
Samurott @ Mystic Water
Ability: Torrent
- Surf / Aqua Tail (HM)
- Ice Beam (TM found in Giant Chasm/Forest Area)
- Slash (Learned at lv. 36)
- Aqua Jet (Learned at lv. 33)
With this moveset, it can do well against foes like Clay, Drayden/Iris, Shauntal's Golurk, Chandelure, and Drifblim, Grimsley's Krookodile, Caitlin's Sigilyph, Alder's Volcarona and Druddigon, and many more.
Swanna
Swanna can be found as a Ducklett on Driftveil Drawbridge. It's another potential Special, Physical, or mixed attacker. It can make a great HM slave and asset to your team.
Swanna @ Mystic Water / Sharp Beak
Ability: Any
- Fly (HM)
- Roost (Learned at lv. 30)
- Surf (HM)
- Ice Beam (TM found in Giant Chasm/Forest Area)
With this moveset, it can do well against the same things as Samurott, with the additons of foes such as Marshall, Alder's Volcarona, Druddigon and Accelgor, Grimsley's Scrafty, and Burgh.
Seismetoad
Seismetoad is a nice physical attacker, but it doesn't have a great physical movepool. It can be found as a Tympole in Pinwheel Forest.
Seiemstoad @ Soft Sand / Mystic Water
Ability: Any
- Surf (HM)
- Earthquake (TM found in Relic Castle)
- Drain Punch (Learned at lv. 44)
- Ice Beam (TM found in Giant Chasm/Forest Area)
With this moveset, it can do well against foes like Clay, Drayden/Iris, Shauntal's Golurk, Chandelure, and Drifblim, Grimsley's Krookodile and Bisharp, Caitlin's Sigilyph, and Alder's Volcarona, Bouffalant and Druddigon.
Jellicent
Jellicent is a bulky, hard-hitting special attacker. It can be found as a Frillish on Routes 4, 17, 18, Driftveil City, and P2 Laboratory.
Jellicent @ Mystic Water / Spell Tag
Ability: Any
- Surf (HM)
- Shadow Ball (TM found in Relic Castle)
- Ice Beam (TM found in Giant Chasm/Forest Area)
- Psychic / Energy Ball / Sludge Wave (TMs)
With this moveset, it can do well against foes like Clay, Drayden/Iris, Shauntal, Grimsley's Krookodile, Caitlin, and Alder's Volcarona and Druddigon.
Carracosta
Another terrifying physical attacker. Carracosta can be obtained as a Tirtouga in Nacrene City.
Carracosta @ Hard Stone / Mystic Water
Ability: Any
- Shell Smash (learned at lv. 40)
- Rock Slide (Learned at lv. 51)
- Crunch (Learned at lv. 21)
- Aqua Tail (Learned at lv. 45)
With this moveset, Carracosta can do well against Clay, Skyla, Elesa's Emolga, Caitlin, Shauntal, and Alder's Accelgor, Vanilluxe, and Volcarona.
Simipour
Simipour can be found as a Panpour in the Dreamyard, Lostlorn Forest, and Pinwheel Forest. To evolve it, you need a Water Stone. Water stone locations are here.
Simipour @ Mystic Water
Ability: Any
- Scald (Learned at lv. 22 as a Panpour)
- Ice Beam (TM found in Giant Chasm/Forest Area)
- Rock Slide (TM found in Mistralton Cave)
- Brick Break (TM obtained near the Pokecenter in Icirrus City)
Basculin
Basculin can be fished up/otherwise encountered on Routes 1, 3, 6, 11, 14, Dragonspiral Tower, Abundant Shrine, Giant Chasm, Driftveil City, Lostlorn Forest, Wellspring Cave, Striaton City, Challenger's Cave, Victory Road, Village Bridge, and Pinwheel Forest.
Basculin @ Mystic Water
Ability: Any
- Crunch (Learned at lv. 24)
- Aqua Tail (Learned at lv. 28)
- Double-Edge (Learned at lv. 36)
- Aqua Jet (Learned at lv. 13)
Note: Alomomola also exists, but it has 40 base Special Attack, and any physical moveset it would have would consist of Pound, Aqua Jet, Double-Slap, Wake-Up Slap, Waterfall, Dive, Frustration, Return, and Facade. It can be a good support Pokemon, though.
Alomomola @ Leftovers
Abilty: Any
- Facade (TM found on Route 8)
- Wish (Learned at lv. 37)
- Toxic (TM found on Route 17)
- Light Screen (TM bought at Nimbasa City Pokecenter)
Hope I helped.