Some form of sandal was the first shoe. Once humans figured out how to make cordage, they strapped protection to the bottom of their feet. It's not much of a stretch to say that something like our current sport sandals has been in use almost as long as humans have been walking.
In 1982, a river guide named Mark Thatcher noticed that the sneakers the guides and customers wore weren't working out too well. He added an ankle strap to a nylon flip-flop and created the first Teva sport sandal. These were lighter, cheaper, and more durable than traditional leather sandals, but the thong between the big and second toes left blisters, so he removed it and added Velcro adjustments. In 1989 another river guide, Mark Paigen, started making a sport sandal with a boot-inspired outsole and heavier midsole that featured a continuous-strap adjustment system and no Velcro, now know as the Chaco sandal. In 2003, Martin Keen added to the category when he introduced his center-lace design sandals with a stretchy neoprene lining and protective toe cap, which more closely resemble an open but protective shoe, but still work like a sandal.
Uses
It's important to note that a 'hiking sandal' is a bit of a misnomer. You can hike in any kind of footwear, or none at all, if you are familiar with the terrain, your ability level, and the consequences of your gear choice. A side-trip off the trail might mean you can't see what is directly in front of your foot, so you kick it. In a boot or shoe, the upper material makes contact and protects your foot, which a sandal can't do. If you are prepared to address the resulting injury without affecting the group, your choice of sandals isn't an issue. If you didn't even bring band-aids, you're going to owe sombody an apology. Additionally, your feet sweat all the time, even in a sandal, and damp skin is more prone to problems. Wearing socks can greatly reduce the chance of blisters and keep the skin healthy, but few folks are willing to combine socks and sandals, which is understandable.
On boats, around water, or for better ventilation, you can't beat a good sandal. Whatever gets in can get out again, and the ventilation may mean cooler feet. With the right construction, features, and fit, you'll forget your feet and get on with the adventure.
No matter where you are in your sandals, don't forget the sunscreen - your feet will need it.
Materials and features
Leather is the historical choice for sandals, but after repeated wet-dry cycles, it can lose flexibility can strength, while synthetics will hold up much longer, though with a more technical or casual look. Synthetics also tend to be lighter and available in a wider range of designs and features.
Velcro sandals are very quick and easy to adjust for daily wear and dry conditions, but continuous-adjust webbing has the advantage in very wet conditions. When saturated, the space between fibers in any fabric fills up and the material expands. This can weaken the grip between the 'hook' and 'loop' sides of Velcro-type fasteners. Continuous webbing, on the other hand, is less likely to slip as the webbing expands in the channels running through the midsole. Also, it's a universal rule in gear selection that fewer seams=fewer failure points.
The outsole of many sandals is now available in non-marking compounds to avoid marring boats decks, and may also be 'siped,' with horizontal razor slits to increase traction on wet, smooth surfaces. Check for this by flexing the shoe agressively at the toe- you should see the slits open up.
Elastic upper and strap sections can make the fit very snug, but keep in mind that you might not be able to adjust the fit as much as you'd like, and there will always be some movement of the foot in the shoe.
Deciding
Once you've narrowed your choices by features, color and material, make sure to get the right size. This can be a bit tricky since some brands make half sizes while others don't. The footbed should match up with the shape of your foot, and the strap length shouldn't be too short or long. Some people prefer a bit more length to protect the toes, but this can also make you more prone to tripping over your own shoes. Your sandal choice will have a strong fashion element, which is fine because if you don't like the way they look, you won't wear them, and what good is that? Whatever your decision, keep in mind that the value of quality will remain long after the satisfaction of savings fades.