Like most cruisers today I carry and rely on a smartphone for many things: email, maps, weather radar, even phone calls (Google it)! However, as someone who enjoys photography, and as someone who frequently sells illustrated articles accompanied by photographs, I have learned both the pluses and minuses of smartphone photography onboard.
The biggest plus is of course that "the best camera you own is the one you have with you." When a great photo presents itself you don't want to be regretting you left your DSLR and its heavy bag back on the boat. Since many of us feel naked without a smartphone, we tend to carry one wherever whenever. This means you won't miss that shot of the amazing sunset, or the funny looking dinghy at the dock, or the cute town ashore. You will have both your phone and your camera with you almost all the time.
I have benefited from this availability many times, and can bitterly remember many missed scenes from the past when all I had were big, bulky, expensive cameras that were likely to be buried in a protective case stored in a locker down below when you saw the scene of a lifetime. Those of us old enough to remember film cameras of the past used to remark that a sure way to encounter a Pulitzer Prize-winning scene was to leave your camera behind, or to be changing your film.
With digital cameras we don't have to worry about changing film, or running out of film, which was worse. However, the smartphone in your pocket is not always the ideal instrument to capture the scene. One huge disadvantage is also an advantage in certain situations. Most phones today have big, beautiful screens that allow for great compositions, if you can see something. Unfortunately, bright sun, shadows, glare, and polarized sunglasses mean that we are often taking photos using the crudest point-and-shoot technique--point the phone in the general direction of the scene and hope you've captured what you want. In those situations take lots of photos to make sure that something is usable.
I find that many boating photos on the water suffer from this problem. Even on days without bright, full sun there can be so much light and glare that using a smartphone screen is nearly impossible. You might be able to see something on the screen if you shadow it, or point the phone in a different direction, but then you're facing the wrong way to get the shot. The bright environment means the photographer only has a vague idea of what she is pointing at, and careful composition relies on cropping the scene later. Take lots of extra photos!
It's a Big Wide World
There's good news and bad news with regard to composition. Smartphones have wideangle lenses, often equivalent to around a 28mm lens for those of you who used 35mm film equipment. Wideangle is great for some things--not so great for others. Typically, a wideangle lens is great for onboard shots illustrating what it is like on deck or down below. But, try to capture that lighthouse you are sailing close to and it will look like you were miles offshore. In general, smartphones are not good for photographing other boats from your boat, or even most scenics, unless there is something really big and really close to your boat.
I have made some interesting shots underway, but most include my own boat in the scene. For example, a wideangle lens can work for sunsets over the deck, or when passing through a big opening bridge that looms over the boat, or when shooting the wide expanse of a crowded mooring field full of boats.
Wideangle lenses are of greater use ashore when looking for telling details: flowers, brickwork, door knockers, etc. But, you have to get really close to fill the frame. They are great for wide streetscapes with lots of buildings and people. You have to be careful with closeups of people because the wideangle will distort faces, making for big noses and goggly eyes. Typically, portrait photographers utilize slightly longish lenses in order to be able to stand a bit further away from a subject and to flatten features, which in most cases is more flattering. Watch out with wideangle lenses not to shoot up at people from close range, which can make for some silly looks. Try keeping the smartphone camera on the same plan as the other person's eyes, or be slightly above them looking down. Again, if you are a lot taller than the other person, or are shooting down at them, you get more distorted looks.
One huge bonus with many phones is that they are easy to hold still and don't create any vibration to mar the shot, while also utilizing digital and other stabilization technology. This means you can take photos in dim light without the use of a tripod, and the best smartphones do a pretty good job of it. Cheaper phones tend to boost ISO (a measure of sensitivity to light) in low light, which results in the equivalent of "grain" that we used to see using fast film. This digital "noise" is not liked by most people, though I have seen some photos that use it to good effect. However, in general, with a top-level smartphone camera, you will find many night scenes come out very nicely. For extra stability try leaning against a light pole, or physically hold your camera still against a wall or table. With some cameras the stabilization technology is so good you can take handheld shots onboard in very dark situations. Try out photography at night with your phone and find out what it can do.