Mineral oil makes a great heat sink but it doesn't have a good way to radiate the heat after absorbing it. You have to pump it through a radiator to get the temps back down. Be sure the electronics aren't on the very bottom in case some water condenses inside.
It depends what your goal is. How many GPUs? Are you doing this for fun or profit?
The only way I could see it being a practical solution is if you're running a large number of GPUs and want them cool and quiet. Then the cost of the liquid heat exchangers in a traditional (tubing) watercooling setup would be significant, and the dunk tank might be nice as a low-noise cooling solution.
Otherwise, if you want a cost-effective way to cool your video card and don't mind some noise, use air. If you want it to be quiet but still reasonably practical, use coolant in a tubing system. It's much less messy. But if you want something that looks cool, go for the tank!