Silhouettes are a lot of fun to create because they simplify scenes by stripping away most textures, colors and surface details and leaving mainly shapes behind. Creating silhouettes is very simple: just position an opaque subject (one that light doesn't pass through) in front of a bright background and then expose for the background. The simplest way to set the exposure is to aim it at the brightest part of the background (in the shot here I used the sky, but the water would have worked as well) and then use your exposure-lock feature to hold that exposure setting. Then simply recompose the scene and shoot.
On most cameras (all cameras--whether compacts or DSLRs) holding the shutter release halfway down will lock the exposure reading. So, if you point the camera at the sky or water and hold the shutter button halfway down to trigger the exposure system, as long as you don't let up on the button the exposure remains locked (and I know that most of you know this already!). The only problem is that when you lock the exposure you also lock focus. If the bright background and your primary subject are at different distances from the camera, you'll have to work around it. One way (with a MILC or dSLR camera) is to switch to manual focus and then you can still lock the meter reading, but you can focus manually--and this is what I do 90% of the time. Another solution (again with a more advanced camera) is to shoot in both manual exposure but keep the autofocus working--so you set the exposure for the sky manually, but the camera does the focusing. Many dSLR cameras also have a separate override that enables you to lock focus and/or exposure separately (see your camera manual for more info).
What about compact cameras? Since most point-and-shoot type cameras have very extensive depth of field (near-to-far focus), focus isn't that huge an issue as long as you're not right on top of your subject. So just leave the camera in the full auto mode (the green mode) and lock your meter reading on the bright background and let the focus do it's thing. If you need more depth of field (DOF) try switching to the Landscape exposure mode. In this mode the camera will automatically select the smallest possible aperture for the given lighting and ISO combination and this will increase the DOF.
One problem you often run into with silhouette photos are "merges" where a part of the foreground is lost in a dark shape in the background. In the shot here you can see that part of the sailboat's mast is lost in the shape of the island. There's not much you can do to solve this problem other than to change your position a bit and try to avoid the convergence.
Read more about exposure and silhouettes on my main site.
Check out my exposure book! If you'd like to learn more (a lot more!) about exposure, be sure to look for my latest book: Exposure Photo Workshop, 2nd edition. It's a comprehensive look at the entire world of exposure and Shutterbug magazine called it "...possibly the best book ever written on the subject."