Unless you and your buddy are lucky enough to have a full face masks with a comms system, underwater communication is difficult and limited to line of sight hand signals or getting close enough to write each other messages on a slate.
The problem is that your buddy isn't always going to be looking in your direction if you need their attention, if you need to point out an amazing find or the fact that you have a problem that needs their help. So, how can you get their attention?
One method that works pretty well if your buddy is swimming parallel to you is to use your torch to either flick the beam in their line of sight to draw their attention or alternatively flash it across the side of their mask. Most peoples eyes are drawn to light and movement so they should spot it quickly.
This method won't work if they have their back to you though, may have limited effect if they have a mask with a solid colour silicone skirt (especially black) and torch light is obviously less visible in bright conditions but it is always worth a go in an emergency.
To help combat the problem a number of products have been developed, all of which involve creating a distinguishable noise. The big advantage of using sound is that you'll be able to attract the attention of a whole group or other nearby buddy pair rather than focusing on just one person.
The simplest and most cost effective solution is a Tank Banger. Essentially a hard object on a bungee cord or rubber loop that is fitted over your cylinder. When you need to get someone's attention you just grab the hard object, pull out and release to make a metallic banging sound when it hits the cylinder. It is obviously most effective when striking the cylinder directly so if you have a nylon or plastic mesh over your cylinder to protect it the tank banger won't be as effective.
Shaker / Rattle
The next step up is a rattle or shaker. Basically a steel tube metal tube with ball bearings trapped inside. When you shake the tube the bearings strike each other and the ends of the tube to create a metallic noise. The shaker is more convenient to operate than a tank banger and can produce a continuous noise for as long as you can continue shaking it.
As the ball bearing are free to move around inside it can cause the shake to create clicks as you move around which may disturb the marine life. If you plan on getting up close to fish or want to remain as quiet as possible you may want to consider a shaker that is fitted with a magnet to restrain the ball bearings when not in use.
The last option is an Air Horn that fits between your BCD inflator and the BCD hose coupling. It uses the low pressure compressed air to create a piercing and very loud sound that can be heard over great distances underwater. You do have to be careful when buying an air horn though as not all of them are designed to be used underwater.
Pre-Dive Buddy Check
It is important to remember that you aren't going to be the only buddy pair or diver that will be using one of these signalling methods so it is a good idea to decide during your buddy check what your signals will be, that way as buddies you'll now what your 'call sign' will be and are more likely to get a quicker response than wild, frantic signalling.