Thank you everyone for your advice.
It looks like I may have dodged a bullet. I got the basses out of the cases quickly and wiped them down. They're in a temperature-controlled room now and I do not see any finish damage on any of them (i.e. bubbling up, etc), knock on wood. I haven't had time to open up their compartments or check under bridges, etc., as I've had so many other personal possessions I'm scrambling to salvage. I don't believe the water got high enough to go past the bridges anyway, but I plan on checking them as time allows. I haven't seen any water pooling under the basses yet since Saturday, so I'm assuming I'm okay for now.
The cases, however, clearly are soaked at the bottom, and I have various cases, gig bags, etc. Unfortunately, the only room I have for storing them right now is the garage, so they're out there opened until I can get them back inside to dry out/de-humidify. So far, I'm not smelling any mildew, so I hope I can stall this a little longer. Which brings me to my other problem: apparently the de-humidifier I already had in the basement got damaged in the flood, as it's kicking on and off but there's zero water in the bucket. I spent all day today looking for a new de-humidifier, and guess what? Everyone's out of stock. :scowl: So I'm not sure what I can do with them at this point beyond doing what I'm doing and possibly blow drying them one at a time.