You are right. My thinking was indeed imprecise here. If we assume that there exists a set of skills such that each skill, if practiced consistently, prevents one from having a specific set of irrational beliefs, then we can impose a partial order on sets of beliefs by observing which set of skills is implied to be absent by each set of beliefs. This partial order can be seen as a ranking of irrationality of different sets of beliefs, and the set of skills shared by a group of people places a lower bound with respect to the partial order, which can then be metaphorically called a "waterline."

Of course, the crucial assumption here is that it is possible for humans to acquire a set of reasoning skills so thoroughly and reliably that they will actually apply them to all issues, no matter what. I don't think this is possible, and with this in mind, I still don't think the "waterline" concept is useful. If anything, it's dangerous because people may fall into the trap of thinking that they are above a certain waterline, whereas in reality, there are issues where due to all kinds of biases even the very basic skills are failing them.