Here is what I used for the basic shower stall.
http://www.lilypodinc.com/
I bought the single stall unit with dome cover and interior seat option. You will definitely want it covered or will be constantly cleaning leaves and pine needles out of it.
I then ran a hose and attached it to this portable tankless hot water unit which I hung inside the shower stall...
http://www.amazon.com/Eccotemp-L5-Po...hot+water+unit
I raised / shimmed the roof on my shower stall 2" to allow the tankless unit to vent properly.
We love this setup and one 20 lb propane tank is all we used all last summer. An outdoor shower is great when you have guests over and don't want to over burden your septic.
We also only use all natural and environmentally neutral bathroom and household products on the island.
Dan