Have you watched Pirates of the Caribbean?
The Undead walk under water.
What happens at the beginning of the New World?
Your ship goes down and you “wake up” in an Island.
That means… everything in New World is actually dead, this is why you respawn after you “die”… It is all an illusion, my friend. We are all dead in the game.
And the water walking is the tip for this mystery.
…
Nah… There is no reason for no swimming. Is really bad to launch the game without.
Vanilla WoW examples;
Portcullis to Undead Stratholme, openable.
Door to Scholomance, openable.
Door to Upper Blackrock Spire, openable.
Burning Crusade; Shadow Labyrinth door, Arcatraz security gate, Karazan had many doors and gates.
Etc.
Lack of time and skill to release the game.
Can you imagine the dumpster fire swimming would be if they added it now? Nothing like a cool swim through Windsward.
Ive seen this before , the attempt at logic. Then the logic fails when someone says ‘When i take off all armour and drop everything I own on the ground why can I still not swim’?
So firing magical fireballs from a staff, zombies, and mystical portals are all believable, but swimming in armour is beyond the realm of imagination?
There’s no swimming because the game is a rushed out untested buggy mess and they didn’t have time to include it would be my best answer to the OP.
Bit fucking mental. What happens if you fall in when you’re like 50m from the shore? 
There was swimming in everquest from day one.
Is this because the water is denser?
You also had to skill it up by swimming afk into a wall for an hour or two 
Even in d&d plate don’t make you less agile seeing it only limits the bonus to dex. It just does not permit great feats of dexterity, but you are still as capable to act and work as any normal human.
Good times
Why is there no swimming? It’s a bizarre omission 
Like, I get it - swimming adds very little and I imagine represents much more work than the gameplay associated with it would justify.
But it’s super strange to just… walk on the bottom of the sea, all the same.
I really wouldn’t be surprised if a future content update added pilotable ships and swimming.
kick and scream and pray, I guess. Most of the time they were much farther than 50m from shore.
A complete suit of plate armour made from well-tempered steel would weigh around 15–25 kg (33–55 lb) . The wearer remained highly agile and could jump, run and otherwise move freely as the weight of the armour was spread evenly throughout the body.
Swimming in full plate is more difficult but it has been proven to be possible by re-enactors and a sailor from the 1600s would be a lot stronger than a modern day re-enactor.
Zombies don’t swim. Everyone on Aeturnum are undead, players too.
paid dlc
Is easier to code a minimap than code swim. Also a sailor that can’t swim is a 2021 hit!
But they could Launch fire from a staff, got it XD
That explains why we are cursed to return to life, and will eventually become Lost.
Are we Chosen Undead?
Are we Ashen Ones?
Are we Tarnished?