Hello, that's great art you have there. And I like of how you hiding their hands, it is kinda cute ^^
I think I can help you a little. The only major issues I've noticed right away is perspective with some anatomy (for example the girl's neck is a little bit longer than it should and the guy's closest chest is smaller). The rest is not so critical and the scene looks solid.
I had some free time (and also was testing my new monitor during) and made some corrections for your illustration, I mean the way I would make this picture it in the exact same composition you have, here look:
Apart from all anatomy and other stuff, you can see I made the foreground a little darker and blurred (so it wont take attention from the middleground, which is our main focus point btw), also faded away the tail into background to make a nice Z-space separation of all objects in the scene. I may failed in that, idk, but you got the idea of how to do that ^^
Oh, btw, since this is underwater, you may also want to add some blue shift (which is always persist on underwater shots), it's gonna look like that:
I hope this will help and keep up with great work ^^
