Just giving someone a ship ful of orcs may not make the best adventure, an interesting fight maybe but not an adventure. So what can you do with them to make it an interesting challenge for the players.
I would suggest just making the characters aware of their existance at first. If the characters are already on one adventure and in a tavern just have some locals arguing about whether there really was an orcish armada that invaded the southern lands. If they are pressed then they say that is what they had heard from a merchant that passed though. Others in the tavern will confirm that is what they have all heard. Now just leave that at that. The locals don’t know any more and cannot confirm or deny anything else.