I took a swim in Sunlight Basin, WY, a while back and rarely go deeper than my thighs any more (4 knee surgeries also helped to dampen my desire to get out just a bit farther).
I was in a flyshop in West Yellowstone a few years ago getting the word about conditions on the Madison; when I asked how far out the fish were, the girl behind the counter said “if your boots tops are wet, you’re in too deep”. Although she was exaggerating for effect (a little, but not too much) a lot of times it’s pretty good advice, especially in the spring when runoff pushes the water levels up.