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"
  ]
}