If a player joins your crew and throws chests overboard, sinks your ship, or kills your pig/chickens that is griefing.
Hostile chat including bad language or racist/derogatory remarks over voice is griefing.
Anything else involving other crews is just simply pvp. If you get chased, sunk, killed, treasure stolen, or attacked for no reason it's literally just pvp. Other crews are playing within the construct of the game. It's the core mechanics. In every battle someone has to lose. If your fun is dependant on winning or achieving your goals at all times every time then you are simply a poor loser. Stop trying to spin it that someone is griefing because you are a poor loser. Please just lose gracefully and stop crying griefer in a pvp/pve game. If you feel some crew is killing you too much switch servers or use it as practice to get better. It's a game.
Edit: Someone mentioned outpost camping. I would also add stronghold camping. Although both are pretty dastardly behaviour they are operating within the design of the game. So neither of those I would call griefing either. Both are just pvp game mechanics. You may not like it but freaking out about it is again just being a poor loser.
I would also like to add I've been a victim of half of this stuff in game. You know what I did about it? I got over it and continued to enjoy the game.