One good thing about swimming recruiting is that by the time a kid is starting his senior year he should have a good idea of where he stands. I believe that A&M's basic criteria for a walk-on is having two junior national cuts, at least it was back under the old junior's system.
There are circumstances that change this either direction. In a strong year the roster just might not have enough room. Or, if a kid hasn't had top level training, but has still done well he might get the benefit of the doubt.
One thing my USS coach told me was to go on recruiting trips even if my mind was made up because you never know what you will like. Some kids just fit in better at smaller schools even if they are fast enough to go to a bigger program.
I'm no expert on this topic, so please feel free to correct me if anyone has better info or advice.