If I had on a pair of underwater infra-red night vision goggles and I looked at a fish, would I be able to see it or does it have the same infra-red signature as the surrounding water?
Well, this is a SWAG but fish are cold-blooded so there probably wouldn’t be that much variation between them and the water around them. Possibly some, but not much.
A bigger question as I see it would be “how far does infrared travel through water?” The longer wavelengths of light (red, yellow, etc.) are absorbed fairly quickly by water which is why everything looks blue below a certain depth. Since infrared is even longer in wavelength I would suspect that you couldn’t see anything more than about 10 feet away from you anyway.