I like cliff-diving, but I am always scared that there might be a rock hidden in the water. If it is a popular spot, then I just trust that someone has already done the work, but what if I am going somewhere new. How do I know where the rocks are?
Always surface dive/snorkel first.
I have cliff dived in two locations - one is normally deceptive: poor visibility, shadows etc but on visual checks turned out to have 100 ft straight down to a sandy floor; the other looked clear and deep but had rocky ledges at about 20 feet!
Considering we dived from 80 - 100 feet, that second location was scary!
Remember that while you may try to go 10-15 feet, as per ShemSeger's post, sometimes you will end up much deeper.
Where I'm from the water is clear enough for you to simply look over the edge and visually check if it's safe, you can see at least 30-40ft deep sometimes if not all the way down to the bottom. The occasions where you need to check depth around here are when you're by a waterfall and your vision is obscured by all of the bubbles. Swimming around the bottom is one way to check depth, but often you don't dive as deep while swimming as you may go after jumping. It's more effective to tie a rock to a long rope and drop it in the spots you're thinking of jumping into. If you use a heavy enough of a rock, you'll easily be able to tell when you've touched bottom, and you'll be able to hear it hitting other rocks as well, the vibrations travel back up the rope and make a deep clunking sound. You only need about 10-15ft of water to jump in depending on how high you're jumping from, less if you've got a soft or sandy bottom.