The rework seems promising. But then again, Pablo revitalizes anything he touches. While I dislike Hydroid's current state, the puddle is an iconic part of his kit The main issues are that (1) It forces Hydroid into a sedentary state and (2) it sequesters enemies, preventing your teammates from killing them. I propose a solution that hopefully resonates with other Tenno here. So:
1. Merge Undertow into Tidal Surge. Tap for Tidal Surge, hold for Undertow.
2. Casting Undertow instead summons a puddle at the location you aimed at instead of directly melting Hydroid and breaking his momentum. Holding the crouch button will cause Hydroid to dive into the puddle and do the same things he does currently. Bullet jump to exit the puddle
3. Any enemies that fall into the puddle are trapped, but allies can now shoot the puddle to damage all enemies within (similar to Tornado).
4. Casting Tempest Barrage directly on the puddle will cause it to grow in size.
Those are my two cents from a non-Hydroid player.