There's usually no need to worry about drowning gliders in a water dish. That is unless you have an extremely oversized one for their needs and fill it so they can fit in and have their head under water. I'm sure this is never the case. However, you don't want it large enough for them to sit in even shallow water, because they could get chilled being wet, but I don't believe that's the case for most water dishes anyways.
The biggest issue with open water sources is contamination, but if you clean and fill daily/nightly, that should be okay. I just like to be sure water is clean obsessively, lol. I despise water bottles now since using a silo for the first time. My glider(s) use silos, but I did add a covered water dish as well, and they used that also.
The most important part is to make sure they have access to water when they want it, and it's kept clean.