I'm swimming deep for something and remembered I had that Killer Whale potion that lets you breathe longer under water so I decided to drink it before diving. While underwater my breath appeared to decline at the same rate it always does and the bar did not seem to be longer either. Under water but close to the surface I decided to drink another Killer Whale potion just before my breath expired to see if that's how it works, like a healing potion but just restoring your breath availability. Nope.

As far as I can tell, the Killer Whale potion does absolutely nothing. I reloaded and tried it several times and I am unable to notice any effect from the potion while it is taken on land before diving or in the water before submerging or underwater with a full lung or almost on empty. Zero effect.

Can anyone else reproduce this? Has anyone got this potion to actually work?

It worked for me when I had to swim through a very long tunnel. You should be able to tell if it's working in certain circumstances, because it also gives you enhanced underwater vision.

It's worked for me. Dived about twice longer.

This is probably not useful but:in a lot of games wearing something different
counter acts something else.

I find most of the decoctions to be not as effective as you'd think or not effective at all. My situation was to do with the Earth Elemental Decoction, which is supposed to "Increases resistance to Vitality-depleting critical effects applied during combat. The resistance level rises the longer the critical effect is applied." However, in my battle with the Arch-Griffin, which can cause you to bleed from a hit, it did the opposite.

When I had the potion active, the bleeding effect was doubled maybe even tripled.. I died very, very quickly even with Enhanced Sparrow and Honeycomb vitality restorative active.. without the potion active, I was able to recover enough vitality to counter the bleeding effect.

Seems kind of pointless to me..

Same here. One thing that works is enhances the vision, but it does not extent by 50% breath. I am in the quest Through Time and Space, 2nd location with gas clouds where it takes your breath. Theoretically it should do the same trick of extending breath, sadly it does not.

Before this quest I tried it two or thrice elsewhere thorough the game, not sure if it worked or not, but seems not even then, and I tested it underwater of course as well. While the enhanced vision works, the extended breath does not work for me either.

One location where I tested it was going to Bald Mountain meeting crows and Imlerith, to get up I had to go through the challenge of retrieving blind woman's coin in the water of drowners and later face the Fiend. And I tested some ocean places in Skellige.

I use mods like FCR3 that fixes bunch of game bugs etc. Have to check the log file if there is a mention of Killer Whale potion fix as well, otherwise I will try to look up on Nexus mods if there is a mod that fixes it as well, since I literally don't see any difference from using Killer Whale besides giving me a brighter vision in the dark underwater, might as well use Cat instead then.

Update: There are no reported issues with it and neither it is in the fix log in description of the made changes.

I think this is because if you just sit underwater the gauge goes down real slow, as soon as you start swimming it goes down real quick... So you're getting maybe 5-10 more seconds, unless you just sit underwater and you get 45 or so extra seconds.