[superhornet59]

i would suggest using an EOS 350D (Digital Rebel XT) by canon. it has amazing high quality, for an amazing low price (near the thousand dollar range by itself, but most are north of 3000) underwater cases are rather common for it.

and if you want to enlarge prints, this will let you make a decent poster, but if its not enough just inerpolate it (the new photozoom pro is the best, but photoshops bicubic is good enough to maintain quality (not exactly detail though)) but the rebels print size should be enough for a poster.