Not allowed water at register

I just started my first retail job this past month as a cashier. They don’t allow us to drink water at the register, we have to call someone to cover us so we can walk to our lockers and drink from our water bottles. Is this normal? My register is also directly next to the window so it gets terribly hot up there. I find myself going dehydrated during a shift because I don’t want to bother someone to cover me. I live in Canada it that helps