Black Manta has atlantian tech, While black panter uses wakandan tech. Both are powerful and exotic technologies.
Manta was able to hurt Aquaman and Panter has vibranian claws.
I would Black Panter on land And Manta in the Sea.
I lean more toward black panter because he has extensive combat training and is enhanced by the fruit while Manta is a human with (navy seal? ) training
Manta will win because Black Panther is not the best. He took out Killmonger and Thanos with help but he also lost to the Falcon, Metal Arm, And Shield and now you expect him to win a fight against an armored man with laser blasters? I don't think it is possible.