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Thinking something like this, would definitely benefit from having some more reef Pokemon. I'd imagine the Gym would be an underwater diving maze, maybe with currents that can pull you down between the different rooms or vents that launch you back up, sort of a mix of the Lavaridge Gym, Route 134 and maybe something like the Pt Hearthome Gym where taking the wrong direction sends you back to the start.
Gym Leader themselves I will admit has some pretty unexceptional Pokemon in terms of a threat to most players but I like Lumineon and think Clamperl should get some love too. I'd imagine other gym trainers would have Pokemon like Staryu, Wimpod, Pyukumuku, Shellos, Shellder, Remoraid, Mantyke, Horsea, Wailmer, Luvdisc, Alomomola, Skrelp & Wiglett just depending on what's available in the region and such. Maybe Dhelmise too for an out-of-type pick.
I'd imagine the gym's town (definitely a town, city would just feel off) would be a fairly laid-back beach town akin to Dewford, Cianwood or Seafolk Village, generally where most fishermen would congregate (probably get the Good Rod or Super Rod from the town depending on which Gym Number it is) and you'd find a lot of sea-themed vendors definitely people to trade stuff like Heart Scales, Pearls and get items like Mystic Water or Soft Sand. Might also act as a sort of ferry if the Gym is intended to be the gateway between early game and mid-game sort of like the role Petalburg Gym has in Gen III.
If permitted new/new-ish Pokemon I'd probably give Corsola a regional form that's basically the opposite of the Galarian crap and have it supplant Chinchou's role for the early gyms. Maybe if the region had a new Water/Grass Pokemon that fit that'd be nice to have.
