I'm not really going to talk much about the movies themselves, but my opinion is that Open Water is a good film, and Open Water 2 is a laughably bad one.
As for the decision to buy this 2-pack blu-ray ($9) or the Open Water DVD ($3) it really depends on you. The video quality of the blu-ray looks just like a DVD (or worse, due to the source material) most of the time. They also have the same supplements. So, really, the decision comes down to two things: audio quality, and whether or not you want to own Open Water 2 to make fun of it with your friends. If you are listening to this movie with a 5.1 surround receiver, I say go with the blu-ray. The lossless audio track does sound pretty good and makes you feel like you're out there floating in the ocean with them. However, if you're listening through the speakers on your TV, you will not hear any difference, so save the $6 and just buy the DVD (unless of course you want Open Water 2.)