Not sure where to start locating the problem.

We had heavy rainfall earlier in the week (Wednesday?), and I hit a spot where water was flowing across the road, probably a good 3-6" deep for about 60'.

After that, the horn sounded weak/muffled, I assumed any residual water just needed time to drain. I didn't drive the car again until Saturday and then I noticed a slight whirring noise but didn't pay much attention to it, not enough to worry about finding the source.

Drove the car again Sunday and the whirring noise was a little louder. When I left work, the car had a little bit of a hard start. On the way home, electrical components kinda went apeshit. Seat belt light turned on while driving, followed shortly thereafter by the airbag light, they'd both then turn off and on at random. As I was pulling into my driveway, the power steering light came on and it was briefly hard to to steer. THEN my gas gauge dropped dead, came back for a second or two then dropped again.

I turned off the car hoping it was an issue with the ECU. I tried twice to start the engine but all the dash lights would just blink rapidly, similar to if you try turning the starter on a dead battery.

The hell do I do now?? My only guess is that I fried the alternator and ran on what was left of the battery.

 

Have you peeked under the hood to see if anything looks amiss? Checked the battery voltage (and/or hooked it up to a charger and/or tried jump starting the car)?

The alternator itself is fairly high up on the engine; I would be rather surprised if it got particularly wet, unless you were fording the water at a silly high speed. Obviously you do have something electrical acting up, though, and it could be in the charging system. Maybe also there's some water causing an additional parasitic draw on the battery, making it run down while the car is parked.

I'm afraid I don't have much useful advice beyond checking the basics that you can easily check. Tracking down electrical problems can be quite time-consuming and sometimes tricky, doubly so without decent service information and wiring diagrams. I wish you the best of luck in solving your problems.