Sandals allow your feet to dry faster after water rides. Since they don't keep your feet enclosed, your feet don't get as warm either.
However, not everyone finds sandals comfortable.
If you've never worn them, and your primary concern is keeping your feet dry, I suggest picking some up and trying them at home, at the mall, etc. You don't want to try a new pair for the first time at a theme park, because you don't want to find out your shoes are giving you blisters 1/2 way through your day.
There's another thread on this board that discusses the best sandals to get, so you may want to check that out.