I've had birks before, and several shoes that were similar construction. My favorite pair were a German shoe (Couldn't tell you the name as it wore off). I wore those many times at WDW and all summer too. I also had a pair of Clark's sandals that were also similar and excellent. Clarks tend to run narrow though.
I like to choose a black sandal with more straps, including an ankle strap. The more straps, the more you can adjust the fit, and the less likely you are to get any chafing. When something starts to rub at all, you can adjust a strap.
Black shoes with more straps also look nicer for evening. I'm not talking galas, but WDW business casual.
Personally, I always bring at least two pairs of comfortable shoes to WDW. Mostly, the second pair are sneakers. I trade off between the two, and in that way avoid major pressure points. Also, I don't like to stretch out my Birks wearing socks.
I'm looking forward to wearing Crocs again at WDW. Actually, I've been wearing mine a lot at home, and have recently found even sneakers to be less comfortable...
I don't really recall getting my sandals really wet. I must have worn them some in the rain. I vaguely recall getting faint black marks on my feet from the bkack leather straps....Crocs are great wet!