|Quite frankly, a small liner is really only useful for catching a very small pee, maybe on the way to the water or something, and you don't want much absorbency, because it'll get heavy.
Even the disposable swim diapers are really just poop catchers. I thought they somehow did more than that (great marketing, I guess), until I got peed on, holding my son, and realized that of course they can't hold pee, or they'd hold water and blow up (the problem with standard disposables). If you read their packages, they even say that they can't prevent transmission of things and to keep your kid out of the pool if they're sick.
From what I've read on this board in the past, the swim diaper tends to run small, but that's really the only comment I've picked up on much--maybe that's because it's assumed that it works?