Hey! This is hard work!!

No easy way out....

Each deck is going to have differing degrees of difficulty in removing it.  If the deck was only half heartedly put down from the factory, with tons of screws, count yourself lucky!  As you can see by these photos, there wasn't much of it that didn't pop up easily.  The hardest part was getting rid of all those screws, (well over 400 of 'em).  The trick is to expose the head without damaging the slot.  I use an old wood chisel to solve this problem, then it's just a matter of applying the screw gun. Several tools will get the rest of the removal job done.  A very stiff, flat, and strong 3" scrapper, a flat pry bar about 18", and a mallet or hammer should take care of most of it.

If the deck was put down properly and is well bonded to the sub deck, your work will be greatly increased and you must use care not to damage the sub deck any more than you have to.  You may have to make use of a router and power planer to get the wood down to the point that a heat gun can be used to kill the bond.

As you can see on this deck, very little adhesive was used, it was applied over gel coat, and in many places the gel coat was not bonded to the underlaying glass.  Originally, this deck had been prefabricated over a fiberglass scrim, then just plopped down over some polyurethane adhesive and had the screws put to it.
While I'm on that point, NEVER trust the bond of gel coat to glass under a deck.  If, as I do, the deck is to be laid with epoxy and no screws, you want to be sure that the teak is either bonded directly to structural fiberglass, (or the sub floor if you had to lay one).  I'll address laying marine plywood sub floors separately and why they may be necessary.

Once the deck has been bared to its base surface, it's time to access any repairs or modifications that need to be done before the new teak goes down.  In the case of this deck, which is a Divinycell core, there was no structural damage or excessive moisture content, so the only modification needed was to remove the gel coat.  There are two principle ways to do this; grind it off or plane it off. If you have access to someone in your area that can plane it, use them!  Grinding gel coat is only slightly more fun than grinding fiber glass!