I can't add much to what's already been said, other than to also say that this comes up a lot for people considering taking a course. It was asked last night at a club meeting by a couple who are considering signing up for their OW class. "Do I have to be a good swimmer?" was the question, and the response from the table was, "Pfffft.... no. Look around."
To be honest... that kinda bugged me.
Do you have to be a world class competitive swimmer to pass the swimming requirement for certification? Most certainly not! You have to be comfortable in the water. You have to have a certain level of endurance and general muscular fitness as swimming works all the major joints and muscle groups. Most importantly, you have to have the knowledge of your abilities to be able to properly pace yourself.
I'm a big girl (trust me, probably in the top 1% of women diving, in terms of BMI). I'm a desk jockey during the day, and am clocking in pretty low on the frequency and intensity of workouts (gym a few times a week to do treadmill, walking the dog, hiking easy to moderate trails). I did the swim test with no problems, switched my stroke on each lap, and took a few deep breaths afterwards and was over it. But I didn't start it like it was a race. I leisurely but consistently swam. I've never had lessons, but feel that more practice and a lesson or two would make me a much more efficient swimmer.
Your swimming skills, or lack thereof, is one element you will consider when you look at a dive and assess whether it is at or above your skills level. Current, surf, if you're being picked up on the spot or will have to swim for pickup -- all those things require stronger swimming skills.
I know I'm going to work on mine to make them stronger. I would hope that anyone worried about their ability to perform or their actual performance in a short distance pool swim will do the same. You might be swimming for your life, or that of your buddy, someday. And waves, current, fatique and all the other things "the real world" will throw at you will make that pool swim look like nothing.