If you are using frozen Mahi Mahi fillets, make sure you thaw it properly according to the instructions.
In a large bowl put 1 Tbsp of salt and the lime juice in the bowl with the mahi mahi and fill it with enough water to just cover the fish. Let stand for 10-15 minutes.
Remove the fish from the water and pat dry, season generously with the spice rub, add the oil and set aside.
Heat the grill on high until it is smoking. Put a bit of oil on the grill and add the fish. Cook uncovered. Depending on the thickness of the fillet the cooking time will vary, for about 1" fillet its about 3 minutes per side. You can bast it a couple of times with the oil and spices from the bottom of the bowl just make sure to blow out any flames that can start from the bottom of the bbq. Remove and serve with tartar sauce and rice or salad.