There isnt a cheap option (I have worked in the camera industry) either a housing as mentiond or second hand sea&sea or nikonas(nikon for underwater) but both expensive (not hobby tools).
If just snorkaling try a disposable 35mm (not APs their crap) goes to abour 5m.
Try ebay for a Minolta 35mm underwater camera.
Had an olympus mju digital with an underwater casing for under Â£260 at cameraworld near oxford circus. Still works fine, but with the constant upgrades and changes about, if you keep eye to ground, might be able to find something a little bit up to date.