The fastest thing on the ocean would be a speed boat. The fastest fish in the ocean is debatable the sailfish has been clocked at 70 mph, and the black marlin has been clocked at speeds of 80 mph.
In special cases certain fish may swim faster or slower, and who can say if those fish that were recorded were swimming their fastest. One thing is for sure, the fastest fish in the ocean, is definitely in the Istiophoridae (billfish) family.