I had a TI-99 as well, floor model with all the goodies, didn't get too much practical use out of it before my first IBM compatible really kicked things off. I watercooled my 4th gen system, poorly, with multiple AIO, and moved on to more custom cooling from that point on. That first AIO card actually did leak, but very slowly.
Luckily the traces are still far more conductive, so you really have to get water going between some specific points to cause damage, usually just results in a shutdown or reboot from what I have seen. Never really experienced an issue in my years of doing it, that leak was actually when a card was on loan and it was the AIO itself with a failed o-ring.
Now if we were talking the early days of water cooling, when voltages were a lot higher and ESD and other protections were not as robust, yeah, not so wise.