This cloak appears to be made from leather.

When it is donned, it appears as a normal cloak until the wearer enters salt water, at which time the cloak of the manta ray adheres to the individual, and he or she appears nearly (90%) identical to a manta ray.

The wearer is enabled to breath underwater and move as a manta ray — 18", see MONSTER MANUAL. 

The wearer also has an armor class of at least 6, that of a manta ray, and other magical protections or magical armor will improve that armor value. 

Although the cloak does not enable the wearer to bite opponents as a manta ray does, the garment does have a tail spine which the wearer can use to strike at opponents behind him or her — damage is only 1-6 hit points, and there is no chance of stunning. This attack mode can be used in addition to other sorts, for the wearer can release his or her arms from the cloak's "wings" without sacrificing movement if so desired.