I have experiance being a Building Inspector for a beach in NH.
Before you close in your outside shower, you should check with the Building Inspector.
The reason for this is:
While an outside shower is meant to rinse the sand off, it is not meant to replace the inside shower.
Is your outside shower plumbed into your homes sewer drain? If not, you should not be using any shampoo or soap because this alows grey water to leach into the soil around your home. This could bring on the Health Officer as well as your State Enviromental Office. (no fun at all)
Also, enclosing the shower may be treated as an addition and have to meet setback requirements or lot coverage. The Inspection Department could fine you or make you remove the enclosure.
What I have found is most beach homes are elbow to elbow with the neighbors and as much as you may get along, they will turn you in.
I once had someone's neighbor tell me about a basement apartment his neighbor had. He asked me not to mention his name because he played cards with him every Friday night. (neighbors are wonderfull).
So, do yourself a favor and check first, you will save yourself alot of money and grief.