If you want to follow tile industry guidelines you'll keep traffic off that floor for 72 hours after the tiles are set with a cementitious thinset mortar.
And for something like installing the water heater, with a couple people and a heavy load, I'd certainly wanna follow that guideline. And even at that I'd wanna protect the floor with some plywood or similar material.
But if you wanna hurry and take your chances, that's up to you.
My opinion; worth price charged.