If you are in fresh water... and it's coming out after each day's use... no, you don't' have to flush it.
If... you sucked sand, or grass into the pump... I would recommend flushing.
If... you run in salt water (or water with high minerals) I would recommend flushing after every use, and use a product like "Salt away" once in a while.  
Going back to the original post... "Running down a dock to turn the water on"??  Is the boat still in the water?  I'm assuming no... but I figured I would ask.
Personally, if you are going to run back and forth... I would go to a hardware store, and buy an in-line ball valve to put on the end of your hose.  That way you can turn the hose on... but still be able to turn the water on and off while at the boat.