What is the best Apple Watch for scuba diving?

Have a look at the related article about swimming with the Apple Watch:

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Water resistance on Apple Watch Ultra models. These models have a water resistance rating of 100 meters under ISO standard 22810:2010. It may be used for water activities like swimming, snorkeling, or recreational scuba diving in a pool or ocean at depths up to 40 meters (130 feet).

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copied from: Important handling information for Apple Watch - Apple Support

well they only make 3 models

likely the ultra 2 you can see the features compared on their

comparison page

Apple Watch - Compare Models - Apple