there are loads of volunteer programs if you look on line, the cheapest ones i've seen are in europe particularly Greece, they may cost a couple of hundred pounds but you could try and organise sponsored events to raise money.
i'm going away for a month this summer to do coral reef volunteering and it cost me an arma nd a leg! but fundraising really helped!
Quest Underseas | Expeditions to Africa & South America this is a quite expensive site about £2,000 for a month (also not the group i'm going with) but they do dive train you, which is useful for marine bio.
don't worry to much about getting experience before university as there will be plenty of opertunities once you get there that will be more valuable and less expensive!