Apple Watch Series 4 has a water resistance rating of 50 meters under ISO standard 22810:2010. This means that it may be used for shallow-water activities like swimming in a pool or ocean. However, Apple Watch Series 4 should not be used for scuba diving, waterskiing, or other activities involving high-velocity water or submersion below shallow depth.
It doesn’t necessarily mean that your watch will fail if its used it for surfing, it’s just with the impact and velocity of the water from surfing could cause it to have issues, but that’s not guaranteed. Other members have used their watches for surfing and had no issues, but it’s at your own risk.