If I have a wet ride the first thing I do when I get home is remove the seatpost and hang the bike upside down in a warm room.
The chances are you don't need to do anything, but as Dan says better safe than sorry and if you want to be 100% there's no water in there, the only way is to remove the BB. Just a couple of points about Dan's instructions - the video is for a seized EBB, in normal removal you just loosen the screws, give it a wiggle and slide it out. Also if you're lazy, you only have to remove a single crank.
It's a good idea to periodically air a frame out even if you haven't been riding through floods, water often finds a way in plus there's condensation. Seatpost and EBB out and leave somewhere warm overnight. Bit of grease when you put them back ensures it's an easy job next time.