Here's my modded Xbox save that allows you to play as a Shark.
1.) Change the save ID's to your ID's and then load save.Code:Download: Here Password: XB36Hazard
2.) You will spawn you in the water as Michael.
3.) Go to Online then Play GTA Online then Go. Once you click go tap "B" as fast as possible.
4.) It should spawn you back in the water as a Shark.
PS: I have used the shark for over 30 mins so there really isn't a time limit.