I did one last year where I swear I finished the bike wetter than the run, on the first lap of the bike it was so strong people were just laughing. Even the local geese got confused and just sat around in the road. Safe to say I learned a few things.

1. Running shoes get stored upside down so they don't fill up (Some might suggest an upside down bag, I tried that but it just filled with water and made things worse, though to be fair my transition stall was in the middle of a stream that had appeared.)
2. Anti-fog on glasses or go without.
3. On the bike play it safe until you know where the grip is. If you are doing laps of a shorter course pay close attention to where the potholes, manhole covers, tram lines etc and other things on the road are, in case it floods.
4. Don't forget wet lube
5. Make sure you bring trainers that don't become roller skates in the wet.

Oh and if you use a towel in transition don't bloody bother. Does nothing and carrying it afterwards when it is completely soaked through and heavy is uncool.