Underwater fog visibility increase is not uniform

When the player's head enters water, a fog effect is applied.

On most biomes, the total visibility extends to 15 blocks from the camera at the start (25% of the total 60), and slowly extends to the total distance over 30 seconds.

However, the distance increase is not uniform. The distance rapidly increases to 60% of its total within 5 seconds, leaving the rest of the distance to be uncovered over 25 seconds.

On Java Edition, the increase is uniform. This was likely a measure to counter the lower visibility distance on Bedrock (60 blocks as opposed to 100).