Shark fin isn't so different from Swing Skate (I like the term shark fin when working with little kids, because they like imitating animals.) You'll need considerably more rotation than suggested in swing skate to pull of a decent shark fin. (I actually prefer a bit more rotation because it amplifies the pendulum-like swing from one side to the other.) If you're not pitched more on your side you will keep falling into flat belly positions. Sinking is not only normal but EXPECTED during this drill simply because a portion of your arm is above the surface. ANY body part held above the surface will make you sink. You can mitigate this by having the 'shark fin hand' deeper (I am almost mid-forearm deep.) When I do the drill it looks more like baby shark fin drill, because the fin doesn't poke out of the water very much.
I don't know that I'd call the drill "abandoned" so much as not the preferred TI drill. I find plenty of value in the drill. and have no reservations using it. If a drill is truly horrible--like the old school "zip your thumbs along your torso" drill--then it should literally be abandoned. If you understand the benefits and potential downsides to a particular drill, then you're probably okay using it provided it offers a meaningful take away.
One thing to be careful with: As you recover the shark fin arm forward, hand at least wrist deep, be sure to have an open and comfortable recovery. Minimize the lifting movement. (I believe that is precisely why Zipper Skate/Zipper Switch were changed to Swing versions--less "up" and more "around" on the recovery.) When your hand is about as far forward as the shoulder it should be maybe 18 inches away from your torso. Do NOT drag your hand close to your body. If your shoulder feels pinched you need to open up the recovery.