They weren't no, but in recent years his biology and abilities have been expanded upon. In the recent New Avengers by Jonathan Hickman we saw him suffer a seemingly killing blow from a knife thanks to The Black Panther initiating a carefully coordinated attack and despite a fate that seemed impossible for him to survive survive it he did.
James Robinson himself showed Namor to have formidable recuperative abilities in his All-New Invaders series, a few short years ago we saw him similarly take severe attacks from Wolverine and Venom in Matt Cherniss' short Sub-Mariner series.
I don't suggest Namor can quickly grow a new head or body like a Starfish might, but rather he can cling on to life even in this dismembered state. As the bottom line is he isn't human. He isn't necessarily just another Mutant either, rather he is somewhat unique...
Would he be much like Superman in that regard, as Supes can survive about anything as long as can access the Sun, and namor the ocean?