Prince of Atlantis: Namor can use the Swim ability. When Namor occupies water terrain, modify his speed and defense values by +1.

Drag to the Depths: When Namor occupies water terrain, if he hits a character that can't use the Swim ability with an attack, after actions resolve give that character an action token and place them in any square adjacent to Namor.

For the Drag to the Depths special power, does the target of the attack have to also occupy water terrain?
No. Only Namor needs to be in the water.

And when I place the target with Drag to the Depths, do I have to put them in a square of water terrain?
No. They only need to be adjacent to Namor. If the there is an adjacent square that is not water, that's a perfectly fine place to put the target.

Does the water terrain need to be printed on the map for Drag to the Depths?
No. Nowhere in that description does the word 'printed' appear. So if you can somehow create water terrain on the map, that will work just fine, too.