I watch series 3 is water proof?

In Apple's own words: "* Apple Watch Series 6, Apple Watch SE, and Apple Watch Series 3 have a water resistance rating of 50 meters under ISO standard 22810:2010. This means that they may be used for shallow-water activities like swimming in a pool or ocean. However, they should not be used for scuba diving, waterskiing, or other activities involving high-velocity water or submersion below shallow depth."

source: Apple Watch Series 3 - Apple

The water lock only prevents accidental screen touches,it has nothing to do with water resistance whatsoever.

Water Lock

Yes it is but before using it under water u have to activate the “water lock” option