What's useful often depends on where you dive.
I think a light is always useful. Even in the daytime, you may want to look under ledges or into cavities. If you dive where visibility is limited, lights are good communication devices.
I almost always carry an SMB and spool. Although you might think them of limited usefulness in simple shore dives, if for some reason you had to come up where you didn't plan to, and especially if you are in an area with boat traffic, it's just a way to warn people there's something underneath them and hopefully make your ascent a little safer. You have to learn to use it, though.
I don't carry a whistle or a DiveAlert or anything of that sort, although I have to admit I've seen them used, and they get the attention of people on shore very quickly. I'm pretty good at yelling, though.
When diving from a boat, it's a little more critical to have ways of getting someone's attention, since ostensibly a simple swim to shore is not feasible.
(Edited because I read X's post after I wrote the original) I like X's ideas, too!