Wow, nice! Thanks a lot, I could have used those colors to up the pixel a bit , oh well, next time 
I made a quick and rough edit to show you how I would do it without adding more colours. However, notice that you haven't really used your 16 colours to their full potential. You have 4 or 5 colours that are basically added by accident, only a few pixels here and there. And you have used 6 colours on the guy in the kayak, when it's pretty hard to notice the difference between 4 and 6. Compare the version on the far left vs the version on the far right. They look almost identical, even though I've eliminated the bright orange and dark blue.
You could easily free up 6 more colours for this piece.
Thanks for the tip! I tried playing around with the waves above the whale but the colors always looked off (the challenge limited the color palette to 16). Have any ideas of how to do this with the colors already in the pic?
Yes, I tried to give it a "rough" water feeling on the top right - it was really interesting working on this.
To me this reads more like a shadow than a submerged whale, because when you remove the highlight of the waves, it looks like something is blocking the sunlight. The brightest parts of the waves would probably reflect almost as much light, even with the whale below.
Cool composition though. Was it your intention to make the waves closer in "frequency" towards the upper right corner, like there's a waterfall just outside our field of vision?
I really like the minimalism in the whale. It takes a second to realise what it is, but wen you do it's shocking.