Just curious, how large is the well? I have a scroll of Enlarge Person, which would give someone twice their height, longer reach, and a +2 str bonus, which I'm wondering if it would be helpful in this case to give better swim checks and hopefully allow whoever to not have to swim quite so deep to reach Namtar, but the well would have to have a large enough opening to admit an embiggened individual.
Alternatively, I also have a scroll of protection from evil, which would give the swimmer a +2 resistance bonus to saves, assuming this is an "effect created by evil creatures".
Could use one or both here.
The pool is about twenty feet across. An enlarged PC would fit in just fine. Also the pool is adequately evil for it the scroll to help.