you shouldn't need anything for a sprint, but if it's a hot day then some liquid/carbohydrate drink on the bike will be sufficient. Do please try the drink in advance of the event to make sure you can stomach it.
+1 to what WildWill said. Unfortunately swimming is the most technical of the three disciplines, despite having a reasonable aerobic base I was just like you when first swimming, gasping and getting out of breath. It does get better with practice.
If you've got (a lot) of spare time you might want to look through the crap swimmers thread
If you do a search on this forum for 'runners trots' you'll find a number of discussions on this topic. Some people have found that changing their diet can reduce the need to go, which might work for you if you don't want to rely on imodium. If you are going to need a pitstop during the marathon a little research will enable you to find courses which are well provided with facilities.
About 1230 today heading south (slowly in traffic in last few miles) towards the QE2 bridge (dartford crossing) - small black car sporting a pirate sticker and someone who looked like they may have been outlawing yesterday
That was me- I'd normally go round the M25 via Thiefrow but the overhead signs seemed to suggest it was a bad idea. I was wearing the event crew T.