We used to have the only dog we've ever seen that could not swim....... She used to try doggie paddling but with front paws out of the water for most of the time, resulting in sinking stern first.
The breed?? Flat coated retriever - some gun dog!
One pure lab who loves to swim, a splab who as soon as the water touches his belly runs a mile and a terrier who gives it a go but hasn't mastered swimming.