Need help with project

[stathis_p]

I was asked by a couple of my friends to help them with a picture they took while vacationing in Jamaica.
They are seen swimming in the very clear ocean, they want me to add a shark under the water!
Anyone knows how to add the shark and blend it in the water, or any tutorial somewhere explaining how to immerse objects under the water.
I did some searching but did not find anything.
Thanks,
Steve

[Dave]

The simple answer is to place a semi-transparent layer of the water over the shark then blend it in with some dodging and burning and a bit of motion blurring .... and of course don't forget the sound effects ....

Posting the image would help

RIP Roy Scheider