Six Great Things to Do in a Shower/Tub

This may be a stereotypical tool for female masturbation, but it can work pretty well for men, too. If the shower head is detachable, you can pull it down and use it in any number of ways. You might try direct, steady pressure against either the front or back of the penis. In a lot of cases direct pressure can be a little strong, in which case you might soften it by placing a washcloth over your penis to soften the sensation. A shower curtain can also work here. But that's just the beginning.
You can also move the head up and down so that the water provides the sensation of stroking the shaft of your penis.
Or you can roll the nozzle from side to side, so that the spray of water washes back and forth across your penis. Personally, I find this motion more enjoyable than either of the first two options.
If the shower head isn't detachable, it's possible to simply lie on the bottom of the shower and enjoy the spray landing on you. Again, I find that this can be a bit harsh on its own, and recommend adding a washcloth to soften the landing of the water. If you don't go that route, you might consider at least using one hand to protect your testicles, which are likely to be the most pained by this treatment.
There are a lot of ways to use the tub nozzle, too, but it depends an awful lot on the shape and size of the nozzle. When I was younger, lived in places where I could lie on my back and scoot up until my butt was against the edge of the tub, with my legs in the air and the water pouring down directly onto my penis. If you can adjust the pressure and temperature of the water just right, this can be absolutely amazing! At fourteen this discovery was by far the closest I would come to the sensation of oral sex for years and years. Unfortunately, as a grown adult I don't really fit under the nozzle of most tubs, so this isn't an option anymore.
In that case, another option to try is to kneel on your knees with your back to the faucet. You may need to be partially erect already for this to work, but if you tuck your penis down between your legs as far as it can go and then scoot back towards the faucet you can intercept the stream of water that way as well. It's not quite as enjoyable as the first option because of the contortion of the penis and because it's rough on the knees, but it's still pretty darn good.