Matt has brought up what I think is an excellent point.
The DM or captain briefs the site. What do you need to know about it, and was that information adequately covered in the briefing?
If the briefing includes swimthroughs or caverns, you need to know how big the space is, whether there are navigational decisions, whether there is ambient light, and what the bottom sediments are -- at a bare minimum. The first cave I swam into, I learned a few things about before I went in -- it was about 20 feet long, had a single bore, had no other exits, and had a bottom made of sand. It was big enough everywhere for a diver to turn around easily. I asked all those questions long before I ever had any cave training, and although I sort of knew what I was doing was probably foolish, I thought it would be okay, and it was. But I ASKED THE QUESTIONS. I often ask questions after dive briefings. I'm listening for certain things, and if I don't hear them, I'll ask about them. Sometimes the guide may have forgotten; sometimes he perhaps doesn't think the things I care about are important enough to mention, or thinks they ought to be self-evident. I have no shyness about asking for more information, if I feel as though I need it. Nobody should hesitate to do that.