First of all, don't rush into anything. It's best to feel comfortable with your lover outside of the shower and the bedroom before you start doing anything new. Try just massaging your lover's shoulders when they're in a relaxed state it'll help you build confidence. If you feel comfortable doing all the things I mentioned fully clothed... you can move on to attempting to the seduction in the shower.
As for coaxing your love into the shower, you can set the scene by creating a love making atmosphere (candles, lingerie etc.). Alternatively, you could also approach them while they are already in the shower... Try asking them to wash your back (or offer to wash theirs) and go from there.
And don't forget you can always talk about it before initiating. I know it can take a bit of courage to ask for your lover to try new things. Explain how you both can benefit from a bath or shower together.
I hope this helps a little!
