I use this pump with my solar water heater, it has a total of 23 feet of suction / lift. It is not submersible but has garden hose connectors on both ends so that it can draw from one large barrel just by throwing a short hose in it and send it out another garden hose at the same rate your house shower can (not the same pressure). It has clamps for connecting to a car battery or jumpmate as the op has. I have also seen this at TSC.
Now, THAT looks like it would have all kinds of wonderful uses...My hubby goes to Harbor Freight (i.e., toy store) all the time, I'll have him look at it.
Ya'll have had some great ideas! I'll consider the fountain pump, too.
Perhaps you could devise a water holding container up there and still provide your shower by gravity. The attic (or roof) could warm it up as well in warm weather.
We looked at pumps this morning at Tractor Supply...there buying chicken feed, anyway...hubby will check at Northern Tool, too. Thanks for all the great ideas!