I would be opposed to 3rd person mode.
When using 3rd person it's too easy to hide behind obstacles, around corners or behind walls and still "abuse" the camera to allow you to see other players while it is literally impossible for them to see you.
With 1st person mode the only way you can see other players is by peeking out from whereever you are hiding, which means they can also see you.
This makes it more fair when it comes to sneaking up on someone or setting an ambush.
I used to play Planetside 1 with 3rd person option and I can't even count how many kills I got from standing behind a corner, abusing 3rd person camera to see enemies approaching and then side-strafe out in front of them and oneshot them to death with a shotgun (the NC J********r triplebarrel shotgun).
In Sea of Thieves, with 3rd person, the number 1 tactic would be hiding behind a corner, use 3rd person, and once they are close you step in front of them with a blunderbus and one-shot them.
I like that it is 1st person. Feels more immersive and keeps the playing field even as far as hiding and ambushing goes so noone has an unfair advantage over others just because of a magic, elevated camera.