What kind of mob are these?  Not specified?  Not a beast, humanoid, dragon etc?  Any ideas?

  They look like 3 headed sea monsters.  They dont show up under any of the hunters detect abilities..just gotta find them

They can be tracked with Track beasts, they're usually close to the shoreline a little underwater.

As I can see now, they are Beasts.

u can locate them as beasts
Very low droprate
Lame quest coz these are not at 1 place
u need to find them by 1 by 1

Wavethrashers are located around 86,36 in the water.

Start at 86,36 and kill all Wavethrashers you see while heading west, make sure you are tracking beasts. Swim around the tip of the coast with the tower while killing wavethrashers. You will soon encounter Young Wavethrashers who also drop the scales. At about 89,19 you are out of the spawn zone, get back on shore and and head to our starting point at 86,36 killing the "beached" thrashers on the way. Now make the trip again, this should have given the beasts enough time to respawn.

It took me two complete trips to gather 6 scales, around 30 min.

can I come up again when I,m down at the shore have tried have to die first XD