mostly intros ...12 megapixels
If the people who market cameras sold meat they'd have spent millions of dollars trying to convince you that a 4 oz McDonald's quarter pounder is a better piece of meat than a 3 oz filet mignon.
"Mostly intros" meaning you're going to take pictures to sell to students doing DSD's? What percentage of them do you suppose will do anything more than look at them on a computer, or possibly a TV? Even if it's a new (and really expensive) 4K TV it can't display all of a 12MP image, because the TV has a lower resolution than a 12MP camera. It's 3840 x 2160 on the TV vs 4000 x 3000 for the camera. Even if somebody makes a 16x20 print, it will still be 200dpi. Good eyesight may let you see individual pixels if you get really close, but for hanging on a wall the quality of the printing will be more important than the number of pixels.
A 15 to 20 MP camera might be better, but it will be based on the size and quality of the sensor, and other things, rather than just the fact that there are more pixels.
All that said, look at the total cost before making a decision. You'll pay the same amount for an Ikelite housing whether you get a G16 or a G7X (there's also a sync cord TTL version for the G16 at +$200), and the strobe(s) will be the same regardless of which camera you get. To a large extent, any upgrade that's still point & shoot will only increase the cost of the camera. The G7X will get you 24mm at the wide end vs 28mm for the G16, and the 1.67x as many pixels will come from a sensor that's almost 3x as big.
As soon as you go dSLR you'll pay a lot more for a housing, so even a modest dSLR starts to get expensive for UW. Unless you're truly serious about the UW photography you'll do I think P&S is a good place to stay regardless of the price.
