It's certainly advertised as a soloable game. eg. Quotes from their Steam store page:
"Stake your claim in this endless open world as you conquer territory, construct ships, search for buried treasure, assemble forts, plunder settlements and hire crew to join your powerful growing armada."
"Wage battle against enemy fleets as you singlehandedly command large ships of war using the captaining system (or divide up to the responsibilities among your trusted lieutenants)"
"Build drydocks and start with a dinghy rowboat, basic raft, tiny sloop or nimble schooner, moving on up to your own versatile brigantine or titanic galleon capable of transporting hundreds of crew and extensive cargo. Name your ship in big bold letters, paint and copy your own pirate flag"
"Recruit real players and AI crew from freeports or rescue seasoned crew from destroyed “Army of the Damned” shipwrecks to man distinct stations on your boat"
"Form a company, claim territory and apply taxation and behavior rules to that which you own: " That's the only place on the whole page where the word "company" appears and that sounds pretty much like a solo activity to me..........