It is common for people to backpack in trail runners and just hike through water crossings relying on the fact that trail runners dry fairly quickly. Similarly backpackers often don't choose waterproof footwear because if these do get wet inside they are very slow to dry.
Beyond that, changing to sandals for a water crossing that will crest the waterproofness of your footwear is commonly done. Some go barefoot although I think it is not wise to risk the injury in the backcountry. People who advocate it might claim the better grip makes the risk less for them.
I like to use light weight sandals I can hike in a little way to handle crossings that are close together. For me that means a heel strap and toe strap rather than toe thong. I've used basic Tevas and Xeros. H2's seem much too heavy to take.
While I can see gaiters preventing light splash over in shallow streams where the water is lower than the boot, I suspect they won't keep knee deep water out even if the gaiters are waterproof. The boots that work for me currently happen to be waterproof so I would not risk getting them wet inside using that method in that circumstance.
I have worn short gaiters with mid boots and light weight gaiters with trail runners both on long day hikes and found them useful for keeping debris out of the shoe but a little annoying to manage. Neither were waterproof and water crossings did not a feature in the hikes I wore them on. With high top boots and long pants I did not find gaiters that useful for 3 season use.
