View all the monotype teams here: https://pastebin.com/S419KvS5

Pokemon Battle Tower Monotype team challenge
WATER team
Version: 1.0
Gold symbol achieved! (https://www.twitch.tv/videos/1649074101)

S (Starmie) @ Lum Berry
Ability: Natural Cure
Level: 50
EVs: 4 HP / 252 SpA / 252 Spe
Modest nature  
IVs: 31 HP / 31 Atk / 31 Def / 31 SpA / 31 SpD / 31 Spe  
- Surf
- Psychic
- Thunderbolt
- Ice Beam
 
G (Gyarados) @ Leftovers
Ability: Intimidate
Level: 50  
EVs: 44 HP / 252 Atk / 88 SpD / 124 Spe
Adamant nature  
IVs: 31 HP / 31 Atk / 31 Def / 30 SpA / 30 SpD / 30 Spe  
- Hidden Power [Flying]
- Earthquake
- Dragon Dance
- Substitute
 
S (Swampert) @ Choice Band
Ability: Torrent
Level: 50  
EVs: 252 Atk / 248 SpD / 8 Spe
Adamant nature  
IVs: 31 HP / 31 Atk / 31 Def / 31 SpA / 30 SpD / 30 Spe 
- Earthquake
- Rock Slide
- Hidden Power [Bug]
- Return
 
Notes:
- Starmie is preferred to Calm Mind Suicune because it speeds up wins considerably
- Choice Band is given to Swampert because it gives a lot of Earthquake OHKOS, plus it can't do much else (Surf/Ice Beam don't complement Starmie and Gyarados' moves well, and Cursepert is just bad)
- Electric types: Switch to Swampert and EQ
- Strong Normal types (Snorlax, Ursaring, etc.): Switch to Gyarados or Swampert, depending on the situation