Phantom can drown

Can perform and confirm this is true. Here's the steps:

1. Make a 3x3x3, 4x4x4, or 5x5x5 hallowed box

2. Flood the box with water

3. Summon a phantom in the flooded box

Edit No. 1 (9/28): No command blocks are needed to perform this. With this note in mind, it's possible to do this in vanilla survival.

Edit No. 2 (10/23): Affects 1.16.200.51

Who said phantoms were an undead mob?

They burn in daylight and are affected by Smite

Still in 1.16.200 betas

Affects 1.16.200.56 Beta

Affects 1.16.100

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Still an issue in the latest 1.17.10 version and 1.17.20.23 beta.

Based on my analysis on the phantom.json files, the minecraft:breathable component has "breathes_water": false, which is the root to this bug.
The solution can be to set the "breathes_water" to true to prevent the phantoms from dying in water.

"minecraft:breathable": {
        "total_supply": 15,
        "suffocate_time": 0,
        "breathes_air": true,
        "breathes_water": true
      }

To be fair, im kinda ok with this lol.