Do you need watercooling to keep them from running too hot?

You can tweak the power limit settings for your application. In many cases you can drop the power consumption (and heat generated) while still maintaining > 90% performance but this will depend on your actual use case [0].

In my experience for many models you can reduce the power limit even further than what has been tested in this guide while barely impacting performance.

[0] https://timdettmers.com/2020/09/07/which-gpu-for-deep-learni...

I use 2x3090 to train large language models, and mine don't thermal-throttle with air cooling even though they're right next to each other. Eth mining does generate too much heat though.

For ML? Nope. I think overheating issues are mostly for mining. I run models and 3D render quite a bit and never ran into problems.