And pistons   

Funny.  I'm in discussion on a separate thread about this topic.  A fully automated version is frowned upon because of impact to server.   However, setup a button, redstone circuit to the pistons, and then have a water flow carry the melon down.
There are tons of (better) designs on line.  A simple one would look something like this:   
(R is redstone and/or repeater as necessary, C cobble/filler, P piston, W Water  D dirt, M melon stalk - pistons/cobble are one level higher than the "M/melon stalk" so that when the piston fires it pushes the melon into the water)
R  R  R  R   R  R   R
R  P  D  W  D  P  R
R  C  M  W M  C  R
R  P  D  W  D  P  R
            |          R
            |          R
            v          R 
collection point   (button)