some sort of bounty system needs to be implemented. Ships that sink others get a bounty on their heads. similar to the reapers mark, they would also be visible to the server (easy to avoid them). It's been discussed many times. Lots of good ideas on this forum.
We cant loose PvP - its part of the core gameplay.
I sailed solo for the majority of my play time, and all I found it takes is a watchful eye on the horizon.
If you see a ship, sail away.
The aggressive players don't like to waste time sailing. If you are too far away, they don't usually bother you, in my experience. Then once you see that the coast is clear - head back to your objective and finish up.
Park on opposite sides of islands to the ships you see.
scan the outposts before heading there.
don't stop using the anchor, so you can get away fast if needed.
You have to be smart about your choices.
Keep watching! - we have a spyglass for a reason :)
I actually like the defensive play that potentially aggressive ships put me in - it makes the voyage feel more rewarding - real stakes.
