I could be wrong, but the demographic for the average diver is a middle-aged male. So for a young guy finding a single attractive woman his own age group and location, it is like finding a unicorn. So naturally he wants to make contact. It is has been a long time since I was 21, but isn't that what 20 somethings do now? Text each other? Friend each other on Facebook?
Maybe flirting is a lost art in the Internet Age. Back in the day, if a young single person happened across someone they found attractive and shared a common interest with, like diving or any other pastime, it was natural to expect a bit of flirting. Where to draw the line--how persistent to be--is a question far older than the Internet, but I suppose it's that much harder to read someone's reactions online than in person.