Make sure you have enough carbs in you before the dive, but avoid simple sugars etc. Ie don't eat sweets, processed wheat etc. But some sports drink or just maltose mixed with water would be good. I would avoid solid food prior to the dive, but make sure I have enough energy.
Maintain a good level of hydration, which is essential to blood pressure. You might take a little salt for example the night before, but I wouldn't recommend it very close to the dive (your body needs time to balance it all out).
This is concerning the big day and maybe a little the day before. Other than that, just eating normal, good and balanced meals normally should do. With the exception that I would avoid onions, garlic and simple sugars for days before the dive.
Now, here's the most important part. If you already have a preparation that you use in training and diving, I would recommend you do that and forget all I have written. It's best to experiment with these things BEFORE the competition, not at the time of...So don't go messing your routine because of something someone wrote in the Internet...But if you do have time to try these things out, then go ahead.
Anyway, the most important things I would say are:
-Proper hydration (to achieve it may take some trial and error)
-The right kind of Carbohydrates
-Avoiding simple carbs
Things to watch out for in addition:
-Of course caffeine etc that might raise metablism and ruin relaxation
-Things that affect bloodpressure negatively (onions, garlic etc)
-Exess of Dairy products (produces mucous and may mess up equalization)
-Also watch out for becoming obsessed about foods. For most cases, normal, balanced food eaten regularily will do just fine.
Talking about pre dive preparation here, not a general diet, which is another topic