There are three dedicated Apple Watch Ultra bands. Ocean, Alpine Loop and Trail Loop. Ocean band is a fluoroelastomer, which is a fluorocarbon-based synthetic rubber. The Alpine Loop and Trail Loop bands are made of high-strength yarns, which offer some stretchability. While you could swim with any of the bands, the band most suited to water sports is the Ocean band. Think about coming out of the water in a fabric band and the lack of comfort as it needs to dry out. The Ocean band will repel water the moment you get out of a wet environment.
I'm a big believe in having different bands for different activities. I own all three of the Ultra-specific bands and wear them based on activity I may be involved in that day.
LD, the Alpine Loop and Trail Loop (specific to Apple Watch Ultra), come in different sizes. The Ocean band you showed, is the only band which is one size fits all.
Here is a link from Apple, with a sizing tool, which I think the OP is referring to --> https://www.apple.com/shop/Catalog/US/Images/bxxd/size-guide_CA.pdf
To the OP. Take a piece of string and tie it to your wrist where you would typically wear a watch, marking where the two ends meet. Then measure the length and convert to MM. That number will permit you to choose the size band best for your wrist size.
OR, stop by an Apple Store and try the various sizes on.
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