Driving through flooded road

I have a 2001 Mitsubishi Galant GTZ, well we had a very bad storm last week which flooded the streets, after things cooled down i went to see a movie and ended up driving into a 1 way tunnel which was flooded, maybe a foot and a half deep, well i got through it fine, but maybe 10 mins later i noticed the "BATTERY and BRAKE" light dim on and off, i pulled over and looked into the engine bay and noticed nothing different, so i continued to drive, i flicked the wind shield wipper blade on and noticed it moved verry slow, after that things went down hill, all my lights, interior and exterior started to slowly dim so, my CD player cd off, and even my tacks on my dash stoped working (everything dropped to 0) (rpm, speed,etc...) so i parked my car and let it sit over for a day, i come back and open the door and notice the door light very dim, tried to start it and it didnt turn over, so i had a buddy jump it, and as soon as i had external power from the jump, everything worked, lights mega bright, my tacks all work, everything was good, but as soon as i took the jumper cables off, everything started to slowly die again, lights, tacks, and all, anyone have an idea??? this is a 6cyl mitsubishi galant, 2001

i was thinking my alternator, because its soo low to the ground, and it could have easly been under the water for the 5 seconds i drove though it, please, any help would be great

Sounds to me that is what you did.....You killed your alternator....bring it to a advance auto store and have them check your battery and alternator for proper voltage and amps just to make sure....They test for free.....Good luck.