Is the Apple 6 series watch water proof? Do I have to do something to turn it on? Can you wear the watch in the hot tub?

The series 6 is water resistant. You can swim in a pool or ocean with it, and not, you don't need to do anything.

While you can use it in a hot tub, its not recommended you do so for extended periods of time since the heat can impact the internal components and the water resistance.

See here: About Apple Watch water resistance - Apple Support

and here: Swim with your Apple Watch - Apple Support

No the apple watch is water resistant;

About Apple Watch water resistance

And I wouldn't wear an apple watch in any kind of water especially hot water,but that's my personal opinion.