If the raw water hose is easily accessible at the thru-hull end, you can pull it off, stick the garden hose in and pack it with a rag. Turn the water on and fire up the engine. Some take it a step further and make a fitting that threads on the end of the garden hose and then has a slip-fit type fitting that goes into the raw water hose and gets clamped in. In this setup is a shutoff. This way the water source is on when you climb up into the boat, so you just need to open the valve and start the engine.