A seal's front legs work just fine as legs.
A “gliding” squirrel hasn't lost the use of its arm, and neither has a bat.
The intermediate need only be of general use for both for it to differentiate into solely or mostly one purpose or the other. Just like a flying squirrel or a seal when compared to a bat or a dolphin. Obviously I am not saying one is an ancestor of another, but the transition from gliding to flying or semi aquatic to fully aquatic are not transitions for which we have no living (or fossil) examples.
As I said. In a question you refused to address.
If you believe all species descended from what few could fit on a boat a few thousand years ago, you believe in evolution that is much stronger and faster than anything supported in the scientific literature.
And what mechanism do you propose stops this ROBUST evolution, hundreds of times stronger than what biologists propose, from moving the animal beyond “kinds”?
Except of course when he's trying to outrun a polar bear on ice, hold young, open a shell,...
whoooo-WEE you're reaching very very badly allmendream!