Communication underwater can be a problem, and is usually restricted to a small set of hand signals. It can be even more challenging to get the attention of your dive buddy, however.
It happens to me all the time, I see something really cool, but my dive buddy is looking in the completely opposite direction. Unless I'm within reach, I have no way to get their attention. This articles addresses presents our options for making noise underwater.
Many stores would be glad to sell you a remedy. Here are a few common choices:
Shaker / Rattle
Probably the most well-known option, many dive instructors carry one of these clipped to their BCD. Most are pretty solid and make a clear sound that can be heard from a moderate distance. Expect to spend about $15 USD.
These devices usually connect between your low pressure hose and your BCD, using the air from your cylinder to make noise. I imagine you can hear these from pretty far, even on the surface. They are not cheap, however, and run around $60 USD.
A pretty rudimentary device, the tank banger is some sort of hard material on an elastic band. You wrap it around your cylinder, and snap the hard plastic / metal against your tank to make noise. I find these a good solution, especially since they are the cheapest of the commercial alternatives, running around $5-10 USD. I've heard complaints that they can break easily, and are sometimes hard to use when the band is still new and stiff.
I lost one of these when it came off while jumping in the water.
The commercial options technically resolve the issue, but can be expensive when you consider the likelihood of leaving it on your rental BCD or cylinder. Here are some options you can whip together at home, and that are cheap enough where losing it doesn't matter.
Shaker / Rattle
At its core, this is identical to the commercial shaker, comprised of some hard metal inside an airtight canister. Old camera film canisters are a popular choice, but are becoming harder to find. A medicine bottle is another option. For the metal, anything from BB's to a simple nut will suffice. Not all canisters will have a way to easily connect it with a BCD, so you may have to improvise, or keep the shaker in your pocket.
This is a great solution, since it doesn't require clipping anything to equipment. A metal band is a great noisemaker underwater when banged against your tank. You don't have to get married to use this one, since any metallic band will do. The noise is softer than the other options, which I like because it only catches the attention of those within your vicinity, but it can be too quiet sometimes.
Here's something we put together the night before a dive in Mexico. It's a hairband with a keychain clipped on it. Buy a pack of hairbands and a pack of keychains to keep with your dive gear and you have the makings for a number of noisemakers, without worrying about losing one. Like the ring, these make a softer noise that I prefer.
Whatever solution you go with, be courteous to other dives. Use it sparingly so you don't annoy everything else diving, including your dive buddy. Check with the divemaster in case she also has one, and that she doesn't have a problem with yours.
And lastly, when in the water, do a quick test so that your buddy learns your "call", especially if there are multiple noisemakers in the water.