Water heater and pump
You should start by closing the taps, replacing the plug in the water heater and switching the water pump on.
For domestic water supplies, once the water pump is back on you can just open and run water through all of your taps. Start with those closest to pump to ensure that any air will be pushed through, avoiding locks in the system. Lastly, drain any water in the tank and refill with fresh drinking water.
If you have plans to cruise you should run your engine up to 'running' temperatures (if a gauge is on board) or for half an hour. Check all around your cooling system for leaks or escaping steam - if you find something not quite right, immediately call out a qualified engineer.
Before you run the engine, check water trap filters and remove any excess water. If water is present or there are signs of diesel bug or jelly, dip the tank to identify the severity of the issue and then treat with a fuel treatment or have the fuel polished accordingly.
To check for water in the tank - get a plastic hose, drop it in the tank. When you feel the bottom, cover the end with your thumb and withdraw the hose. This will give you a good indication to what is at the bottom of your tank, and the amount of water present.
There might be a build up of water in the engine bay, particularly if your boat has a cruiser type stern. Visually check the bilge water before pumping it out to ensure there is no floating oil. If oil is visible, you should add absorbent oil pads to the water surface, to ensure that the oil is soaked up before pumping out the bilge water.
Remember to service the engine, LPG and electrical systems, fire extinguishers and escape hatches.