Water doesn't magically arrive at the sink or shower; it is pumped from the tank that you filled via on onboard pump, which pressurizes the water, just like a well pump does, out in the country. Is the breaker turned on for the water pump? You'll be able to hear the pump if it is running (as long as your engine isn't running at the same time.) When it first starts it should run for quite a while, while it fills up the water heater, brings the lines up to pressure, and then shuts off automatically. Could be as much as 10 minutes, the first time. Then after that, it should come on whenever you're running water, showering, or whatever. When you turn the water off, it will bring the lines back up to pressure, then shut off - usually in just a few seconds.
Most onboard toilets flush with raw water from outside the boat - lake, river, ocean, etc. The toilets have built-in pumps to bring in the outside water and send it off to the holding tank.