Here is the setup I have used in my long (1-3 week) treks in northern Scandinavia:
For shorter crossings
(or if the water is warmer)
Good, sturdy sandals (e.g. Teva)
Zip off legs of your pants, or remove them completely to keep them dry
Keeping socks on can help some (but not a lot)
(Fording stick and/or trekking poles)
IMHO the risk from sandals is not ...
Many rivers are glacier-fed and some are quite wide, so the risk for hypothermia is real.
I doubt that. A river is at most a couple of degrees below 0°C. Suppose it takes 10 minutes to cross and it's knee-high. That will not lower your core temperature significantly. The biggest danger is feet becoming numb and you falling into the water, which can be ...