For the Amberjack this sounds really good:
Grilled Amberjack Tacos

For the Mackerel:
First, don't freeze them or they are really bad...
Second, Definately grill them outside (or the house will smell like fish for days!).

place filets (skin side down) in aluminum foil large enough to make packets.
sprinkle top of fish with olive oil & S&P ( and optionally, some Old Bay) to taste.
place 10 or so raisins on top of each fish.
top with thin onion & lemon slices.
seal off each packet.
grill for about 5-10 minutes per side (depending on thickness) over medium heat.
DISCARD THE RAISINS!!!
(they are only there to "soak up" all the fishy oil in the Mackerel).
I got the idea for using raisins in oily/fishy tasting fish from a bluefish dish we used to make when I worked at The General Wayne Inn in Southold, NY back in the 1970's...
They really do soak up the oils...

Serve with tartar sauce and lemon wedges and maybe some corn on the cob and grilled pineapple.