Find a Shower Timer of many different kinds. Some are digital atomic clocks like the one shown here, and some are sand hour glasses. Find all types through the following link:
Browse all Shower Timers here.

The Shower Timer shown here has many customer reviews to read through and most of them lean positive. This one is an atomic clock that you don’t have to adjust manually. The suction cup holds it up to the tiles and it is waterproof.
Read more about them, or buy them here.

Pros: 

As I mentioned, the reviews are plentiful and mostly positive. Adjusts automatically using a broadcast out of Colorado, so it is always accurate to your location. Also displays date, and also includes an alarm.

Cons: 

A common complaint pertains to the suction cup not holding properly, but many other customer reviews have pointed out that the suction cup works fine as long as you follow instructions properly. Read all reviews good and bad and be your own judge. You can do this through the links on this page.

If you are looking for a very basic and inexpensive type of shower timer, you could look at these very simplistic hour glass styled options. The one you see here is a 5 minute shower clock, which is obviously waterproof. Use it to save water and heat, or to teach kids about efficiency or water usage. It’s also a good solution for needing a timer in any wet environment other than the shower. Click the image of this blue shower timer to find details about this one, or to browse through many other timers very similar to this one.

This is another somewhat inexpensive and basic type of shower timer or kitchen timer. This one is the digital shower coach shower timer. This is a suction cup backed timer that is blue and shaped like a water droplet. Three simple buttons to operate it. Many customer reviews to read though about this one, however, there are mixed reviews, with most of them being negative for a variety of reasons. Click the image of this digital timer to see those comments, or to see many other similar timers.