Superhuman Swimming – The ability to swim at superhuman speeds.
Marrina (Marvel) can swim 48 miles per hour and even faster if she sheds her skin and uses a frictionless skin underneath (Official Handbook of the Marvel Universe V1 #7).
Namor (Marvel) has this power as do all members of his mother’s race Homo mermanus. According to the Official Handbook of the Marvel Universe V1 #10, Namor can swim 60 miles per hour. This means that according to canon sources, Aquaman can swim faster than Namor.
Olympic level humans can swim at 1.64 meters per second, or approximately 3.7 mph (Toussaint, HM & Truijens, M 2005, ‘Biomechanical aspects of peak performance in human swimming’, Animal Biology, vol. 55, no. 1, pp. 17-40.).
The fastest fish in the world is the sailfish which can travel 68 miles per hour. The means Aquaman is can swim faster than the fastest fish in the world. Namor swims more or less at the same speed as a sailfish (60 mph vs 68 mph).
In the video, Justice League: Throne of Atlantis, Atlantean warriors are presented as swimming so fast that when they attack that Cyborg can barely perceive them. Cyborg later comments that the Atlanteans warriors apparently do not only rely on speed but use the equivalent of underwater ninjutsu, the exact word ninjutsu is not used by Cyborg, to hide in the shadows, that are probably more prevalent underwater. Below is the actual fight between Cyborg and the very fast Atlantean warriors.
Justice League: Throne of Atlantis – Cyborg Attacked
The point this video makes is that underwater denizens do not just swim faster but move faster underwater overall and that this would give them a crucial speed advantage in underwater combat.
Namor uses his superhuman swimming ability offensively in the Incredible Hulk V1 #118 in order to create a whirlpool. The Hulk has been given a tablet that lets him breath underwater in order to make this underwater fight possible.
Namor is basically employing a speedster trick underwater. Quicksilver uses his super speed to do the land version of the whirlpool trick in Amazing Spider-Man V1 #71.
Aquarus (Image) can swim about as fast as Namor using conventional means but uses something called supercavitation to swim 500 miles per hour (Official Handbook of the Invincible Universe #1).
However super cavitation is not swimming in any normal sense of the word but would be yet another superpower and there is research to apply this phenomena in real life!