Could anyone recommend a mini (approx. 2mm x 2mm) waterproof USB infrared webcam for Raspberry Pi. This will be used for aquatic life monitoring. Any suggestions would be appreciated as well.
2×2 mm waterproof casing means likely a 1x1 mm sensor, which means terrible image quality and low resolution. Are you sure this is what you want?
In general, tiny cameras measure around 6×6 mm. In order to find them, you need to use the term “snake camera,” “endoscope,” or “borescope.” The last two terms mean different things, but they seem to be used interchangeably on e-commerce sites.
The problem is that I couldn't find any infrared cameras. A bunch of websites mention infrared endoscopes, but I don't think the products are trustworthy. Here's one, and here's another one, both having a 5.5 mm diameter. While the “infrared” term is used in the titles of both products, I'm not sure that the sellers meant that those are infrared cameras.
Maybe you should consider changing your requirements, such as using an ordinary infrared camera module for Raspberry Pi and put it outside the water tank or in a custom-made waterproof case...