Like the $100 ones you can get on Amazon. Experiences?  Opinions?

Maybe link an example?

Sorry, here's an example. There are a ton of different brands but they all seem to be basically the same thing.

Salmue Watch Waterproof Test Tool, Watch Case Resistance Pressure Tester, 2 Watches Watch Case Resistance Measuring Tools Water Resistant Tester Tool for Waterproofness Testing https://a.co/d/bQu9nS1

Interesting. Not sure I’d want to do it myself. Seems you’d have to risk damaging the watch in order to determine if the seals are good. I think I’d rather have somebody insured to do that and have them take on the risk. Sure, four an Invicta why not. Doubt I’d want to throw an premium watch in there personally.

Yeah, that's what I thought as well. From the videos I saw from watchmakers that use it they have said that water can't get inside because of the air rushing out. Now if you keep it in there it'll eventually flood.

I won't put a Rolex or Omega in there but a watch I can take the back off and am comfortable removing the movement on would be safe enough I think. That would probably be the one I actually take in the water anyway.

I'd have a legit watchmaker test it but it's a hassle since they aren't conveniently located, there's a multi week wait, and it's still like $50.

A cheap tester can give an indication of a leaky seal, but you have to do it right.

The correct procedure is to hold the cased watch up out of the water and pump it down to whatever depth you're testing for.
Hold it above the water for a couple of minutes, then lower into the water and release the pressure.
If you see any bubbles, quickly pull it out, it failed the test.

What's happening is the air pressure will leak into the case through a bad seal and pressurize the watch.
When it goes into the water and the pressure is released, the air in the case will leak out causing bubbles.
If you don't just leave the watch under the water, the escaping air will prevent water from entering.

Unless you have a lot of watches to test or you use them under harsh conditions that can cause seals to deteriorate, the money really isn't well spent.  All you need to do is have the seals replaced every 3 or 4 years, depending on the conditions you use them under.
One way people ruin seals is taking a hot shower or sitting in a sauna.  The heat causes the neoprene seals to get hard and fail.

Just don't do what one person did.  
He put the crown and back on the empty case and did the test.  The pressure blew the crown out since there was nothing to hold it.
He thought the case was defective until I explained it.

Great info, thank you. I ended up deciding not to get one in the end.