Eliminate the "I don't feel clean enough" excuse!
Standing-wet-doggy - She faces wall, arms pressing against it, back arched, bum-out! Hold her hips and control the thrusts. Remember to help her balance, depending on the type of shower interior available make sure you even-steven both of your positions, during the down-pour.
Slippery-oral - You're both clean and fresh, so why not take advantage on the situation by ducking down for some oral fun. Unlike when it's dry, you can't place a pillow under your knees, but you can squat instead or simply bend slightly while the receiver does a leg lift on a shower wall. Don't forget to come up for air!
Misty Masturbation - two words. Detachable Showerhead! The beauty of getting yourself off in front of each other in the shower is that cleaning up becomes quite graceful and easy.
Play in the mud - Showers have marvelous cleansing powers, and therefore gives room for anal adventures to be performed with less self-consciousness for both parties.
Soap is not lube - and is for external use only. Neither are shampoo's, conditioners, shower gels and shaving creams. Water does tend to dry up natural lubrication, so be sure you're prepared with silicone-based lubricant. Be careful not to slip!
Remember to get your foot work right while thrusting, perhaps even invest in some adhesive grip mats. Handle lubricants with care, and most of all have a "wonder fall" time.
Showers making dirty ..clean! Have fun!