This is something I've pontificated (is that the right word?) about for years. First you have to define "very comfortably". My definition is simply using a proper stroke. This is something I only saw students do on occasion while I was assisting OW courses. More than not, the head came out of the water side to side with each arm stroke, among other technical errors in form. I had trouble with the DM 400 meter swim because my mechanics were off, having not done any real swimming for 35 years, though a ton of snorkeling in all kinds of conditions--WITH FINS, which means really nothing much at all to do with proper swimming. Also noted that I was on the high school swim team circa 1970. Having said that, I can assure you I was extremely comfortable in water since maybe age 7. I did get advice from my competitive swimming brothers, who fixed up my problems, which basically were poor breathing technique, too any kicks per arm stroke, and poor arm stroke technique. You forget a lot in 35 years. I "gutted out" the 200 in OW and was definitely not comfortable about my swimming. Eventually I scored fine on the DM 400 when I fixed up my technique (scored 3 out of 5, or about 400m in 12-13 minutes--not Olympic Gold, but pretty good at age 56 after almost 40 years of no swimming). All of that stuff has very little to do with diving. Though I actually do agree that you should be able to swim to at least some extent and at least KNOW about correct technique prior to taking OW. I've seen some pretty interesting cases with students when I've shaken my head.
Sorry to rehash all that old stuff.