I'd bring over Tartaglia, because I think he'd be a cool addition to this game, and probably have good chemistry with Sampo or the Trailblazer.

He'd be an Ice (closest to water) Erudition character. Skill would deal damage to a single target, and applies a permanent Riptide debuff. Once an enemy with Riptide dies they would explode, and enemies adjacent to it would take damage and then also be affected by Riptide. If those enemies die because of the Riptide application damage, then they'd also suffer a Riptide explosion, and restart the whole process.

His Ult would summon his giant whale, damaging all enemies, and dealing bonus damage to those affected with Riptide.

His passive would probably be a team buff, like his insignificant Normal Attack +1 in Genshin. Here, I'd probably make it to where the team itself deals bonus damage to Riptide targets, or those under a certain threshold of HP.