All Apple Watches are water resistant, but not waterproof. Water resistance isn't a permanent condition and may diminish over time. Apple Watch can't be rechecked or resealed for water resistance.
Apple Watch Series 2 and newer may be used for shallow water activities like swimming in a pool or ocean. However, Apple Watch Series 2 and newer shouldn't be used for scuba diving, water skiing, or other activities involving high velocity water or submersion below shallow depth.
For more info, see:
About Apple Watch water resistance - Apple Support