Spin jump to the left through the transition. Bonking the left side of the door frame works but is not required. Hold left through the transition to avoid the fish and land on the platform to the left. Then reach the platform above either using a crouch-jump down-grab or by freezing the fish.

Entrance condition:

{
  "comeInWithPlatformBelow": {
    "maxHeight": 6,
    "maxLeftPosition": 2,
    "minRightPosition": -2
  },
  "comesThroughToilet": "no"
}

Requires:

"canCrossRoomJumpIntoWater"
{
  "or": [
    "h_canCrouchJumpDownGrab",
    "canUseFrozenEnemies"
  ]
}

Dev note: This doesn't work with passing through the Toilet (e.g. from Climb) because you would spawn higher and not be able to avoid the fish, given that no horiziontal speed is carried through the transition here.