How can I set up my apple watch S5 to waterproof for water and run race

The Apple Watch is water resistant - not waterproof. All you can and should do is in this article:

About Apple Watch water resistance - Apple Support

If you are running a triathlon or some form of one you are quickly going to find that the Apple Watch is not your best choice for this type of activity. Look at the units from Garmin specifically designed for that form of competition.

The only proactive "water mode" you can enable is to enable the water lock mode: Scroll up on the watch from the bottom and click on the water droplet. This mode is also automatically enabled if you start a swimming workout. This will lock the screen so any random touch/water does not get interpreted as a touch and then, when you scroll the crown, it emits a sound through the speaker to clear any water. As long as you dont have a need to interact with the screen during that time, the watch is ready for water use. Just keep in mind that any touch is ignored until you scroll the crown and hear the beeps it emits.

The Apple Watch is Always water resitant. The Water Lock just turns off the touch screen and side buttons so that accidental touches, maybe by water splashing, will be ignored until you twist the digital crown.

The only issue is the length of the race. If you are participating in a full length ironman (2.4 mile swim (3.9K), 112 mile bike (180.2K), and 26.2 mile run (42.2K)), you may run out of battery charge before you finish the race. If the race is shorter, such as just a few hours, then the Apple Watch should be fine.