Exit condition:

"leaveWithGrappleSwing": {
  "blocks": [
    {
      "position": [-1, 5],
      "environment": "water",
      "note": "The closest solid (non-crumbling) Grapple block left of the top door"
    },
    {
      "position": [12, 5],
      "environment": "water",
      "note": "The closest Grapple block right of the top door"
    }
  ]
}

Unlocks doors:

{"types": ["ammo"], "requires": []}

Dev note: Other Grapple blocks could be used on the left side, but there's no apparent reason to.