How do I make my Apple SE watch waterproof

You can't,apple watch is water resistant not waterproof,see this page;

About Apple Watch water resistance

The Apple Watch is ALWAYS water resistant. The Water Lock feature just disables the touch screen and side buttons so you do not accidentally "Butt Dial" someone while swimming because of water splashes or flexing of your hands presses against the side buttons.

If you really want waterproof, then you can get a case that claims to be waterproof. But best to check their claims, as I'm sure there are limitations in the fine-print