I did this for years and years when I had a plastic greenhouse, and it was a delight to shower out there among the plants in the warmth of the afternoon. I think the plants enjoyed it too.
Even better than a hot shower in the greenhouse is a hot tub! A friend of mine has this, and fires it up on the coldest nights with a woodstove. The added benefit is he leaves the hot water in the tub overnight to help heat the greenhouse.