Originally Posted by Grant
To work the abs even more do the dolphin drill on your back with the arms either above the head in line with body or along the sides. Also vertical dolphins with the arms folded across the chest. Can progress by having the arms out of the water to varying degrees until they are straight above the head. Work up to this gradually in terms of time and difficulty.
Depending on how fluid your dolphin kick is on your back, I would suggest using a mixture of fins and no fins.
If you are doing dolpin kicking on your back with your arms either by your side or in a streamlined position(like swimmers do when they push-off the walls at the start of swims or after turns) I would say don't use fins.
If you want to experiment with degrees of difficulty, like grant mentioned above, I would recommended fins until you develop the core strength to start doing a few lengths without them.
I have done some of the positions grant mentioned above, they really do work your abs. If you don't have history or current shoulder issues, these will not put any stress on the joints.