The wearer can release his arms from the cloak without sacrificing underwater movement if so desired.
So can the wearer use his arms normally while in "Manta" form? Can he use a weapon or shield, sort of like a Veritech Fighter in Guardian Mode?
Can he use his arms and hands for other things, like grappling or skills?
It would seem. It says it is like beast shape, which would indicate no normally. But the last paragraph says you can use your arms,so the exception to the general rule is made.