behavior_packs/vanilla/spawn_rules/dolphin.json

{
  "format_version": "1.8.0",
  "minecraft:spawn_rules": {
    "description": {
      "identifier": "minecraft:dolphin",
      "population_control": "animal"
    },
    "conditions": [
      {
        "minecraft:spawns_on_surface": {},
        "minecraft:spawns_underwater": {},

        "minecraft:weight": {
          "default": 7
        },
        "minecraft:herd": {
          "min_size": 3,
          "max_size": 5
        },
        "minecraft:density_limit": {
          "surface": 5,
          "underground": 0
        },
        "minecraft:biome_filter": [
          { "test": "has_biome_tag", "operator": "==", "value": "ocean" },
          { "test": "has_biome_tag", "operator": "!=", "value": "frozen"}
        ]
      }
    ]
  }
}