Positive pressure means that any water getting into the mask would be flushed out. This first-person account seems to clarify that the kids were almost always on a stretcher while on land. But what about the underwater portions? I can't even imagine as a kid (or even adult) diving into the equivalent of no-visibility iced coffee. This Nytimes detailed recap seems to indicate the stretchers were also used during the "sumps" (underwater portions)
https://www.nytimes.com/2018/07/12/world/asia/thailand-cave-...
The Thai Navy Seals released their video of the unprecedented rescue operation this past week. It has no sound, and it is a perspective into how many individuals contributed to the success of the team effort. You can see the sked described in the article (a stretcher used for search and recovery) and the zip line/winch/pulley systems that were used throughout.