Defrosting meat/fish in warm water
This in general can be unsafe, because the temperature of the warm water will be in the danger zone. That means that you'll be holding the surface of the food in the danger zone for the duration of the defrosting. Since you don't want to hold food in the danger zone for more than a couple hours, and that's cumulative over the whole process from fridge to eating, this additional time can easily take you from safe to unsafe.

If it's a really small piece like a thin filet it might only take 15 minutes, so it'd be fine, still easy not to go over two hours in the danger zone. If it's very big, though, it could start to be an issue. You can mitigate this some with warm running water or a circulator, but that only goes so far. And in either case you have to be very careful to keep track and make sure the defrosting is fast enough.

Cool running water, on the other hand, is a recommended method. It still works quite quickly, and if the food does enter the danger zone it's at least on the low end of it, so it's not quite so risky.