OK, I was considering asking folks why anyone would get so revved-up about the one earthly terrain that adventurers are least likely to spend any great deal of time exploring or adventuring in, but I figured I'd let it pass because I wouldn't receive satisfactory answers.
But now I have to take the bait. How the heck does water qualify as having "the largest appeal to the masses"? A party trekking through desert or tundra has a certain heroic appeal to it. I can paint a cool picture of that in my mind. But a party trekking across an ocean floor with schools of fish swimming around them, and checking their watches every few minutes to see if it's time to take another swig of their water-breathing potions? That just strikes me as an absurd visual. I don't see that as having the greatest mass appeal.
Water is an obstacle that separates point A from point B. Sure, your ship can attacked by krakkens and pirates in transit, but that alone doesn't merit an entire book IMO.