Apple Watch Series 7 (GPS + Cellular) and Apple Watch SE (GPS + Cellular) can use a cellular connection for Emergency SOS.
The irregular rhythm notification is not designed for people who have been previously diagnosed with Afib. The ECG app is available on Apple Watch Series 4 or later (not including Apple Watch SE).
Series 7 is also rated IP6X dust resistant. However, they should not be used for scuba diving, waterskiing, or other activities involving high-velocity water or submersion below shallow depth. This means that they may be used for shallow-water activities like swimming in a pool or ocean.