WE all know this is a touchy subject. Everytime Rare has tried they have been met with the biggest opposition to a real problem.
The issue isnt that people board your ship, Its not that they kill you or even double gun, although, I find the super fast 2 shots to be less than sporting.
There are 2 main issues.
The first is the biggest problem.
CANNONBALLS SHOULD HAVE FRIENDLY FIRE
The most obnoxious part of the PVP in this game for any player is that the safest place in a fight is the enemy ship. you are safe from every cannon shot there. you only have to deal with a crew that is generally distracted by well, cannonballs knocking them around while you stand there freely shooting and slashing as if there isnt a ton of lead falling from the sky around you.
This will balance the tactic that rare has been trying to balance since the arena came out.
The boarders have to deal with their own team knocking them around as much as we do. This is more than fair. Honestly, you standing on my boat and a cannonball hitting you in the face and killing me is entirely unfair and unbalanced.
The other is just, the ability to heal while getting slashed and shot but, that can be mitigated by hiding all your fruit.
Edit
Let's also realize how this will balance galleon vs sloop as well. Having 3 guys on a galleon pummel the deck of a sloop while you're boarder stands there is unbalanced too and I'm a galleon guy.
