Don't each too much protein and fat of the morning of the comp - there's more chance you'll get a bit of acid/reflux with fatty foods (fine for the week leading up to it though). Also, protein and fat takes a lot longer to digest so you'll have that full sickly feeling for longer if you eat a fatty brekky.
That said, a couple of eggs and a little bit of bacon has always got me through a cold session in the quarry. Mind you we were eating it about 3 hours before diving and only staying in the water for about an hour. The fattier food seems to protect you a bit better from the cold.
In the few days leading up to it eat plenty of carbohydrates like Erik said - pasta, rice etc. Not cream based if your sinuses clog up with dairy.
But for a long day's diving i'd eat plenty of grain bread with honey/jam and maybe some scrambled eggs with a little bit of bacon. If you eat around 8-9am you'll be fine through to 4pm because you rarely think about food when diving, especially in a comp too. Lower GI foods are better because they release the energy over a longer period, so grain bread instead of white, apple juice instead of orange etc. Bananas are great too - lots of slow release energy (only don't take them on the boat because the superstitious captain will be sure to throw you off!)
And Erik is bang on the money with the fluids - sport drinks if you have them, otherwise apple juice and plenty of water. I used to drink a hell of a lot of water before a spearing comp because I never took any with me on the dive. It would make me uncomfortable but water goes out of your system pretty quickly so it wouldn't be for very long.
Sodium/Potassium retention is a big issue too from sweat and immersion diuresis. Sport drinks are great for this. If you don't have the sport drinks then Bananas also have lots of potassium, and have a bit of extra salt on your eggs in the morning.
And don't forget to top up when you jump out of the water, sometimes it can be ages until you get home or get to a shop so take some bars or something with you.
If you're a coffee fiend then a cup in the morning will be fine. As a student I was told by a wise man that when sitting an exam you should be on the same drugs as you were on when you studied for the exam so your body didn't feel radically different. Same principle with training - if you're used to something (like coffee) then your body will expect it so don't deny yourself. There will be absolute minimal effect for something like a spearing comp.
Hope that all helps! Can't help you with the comp itself, i'm a crap spearo and never actually weighed in any fish when I attended comps
Cheers,
Ben
