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.
Some of the things the orcs like to do is sail into harbour, launch fire arrows at every boat they can reach and sail out again. It doesn’t do them much good but they do enjoy the power trip and the thought of the destruction. It isn’t all senseless violence though. If a ship puts to sea in pursuit then the orcs will lead it out until it is isolated and then turn on it. When it comes to hand to hand fighting then the orcs are a savage and effective crew. If they capture a ship in this way but it is too big for them to crew then they will either scuttle it in the mouth of the harbour it came from (towing it along) to serve as a grim trophy or if the wind is blowing right they will set it on a ramming course back into its home harbour to try and wreck another ship. They will hang some of the crew from the spars as a grisly bit of theatre. When the ship collides with another vessel or the harbour wall the bodies will dance and swing on the ends of their rope.
It is after a ‘wrecking’ like this that the orcs have tried to demand a ‘tribute’ from the port to be left alone. Failure to pay will often mean that the orcs will moor their boat in a cove or bay near by and then raid from the landward side burning farms and out lying homesteads. The orcs like this as it provides them with fresh provisions, coin, odds and sods of equipment and a good fight on solid ground. They will often kill and raid in an arc from their hidden ship in land but not burn. They will attack and press on all though the night. At the first sign of pre-dawn they start to torch their victims properties and retrace their path burning each site as they go. This tactic means that men sent out to investigate the first plume of smoke are heading in the wrong direction to find the orcs. The orcs can also stash the booty from each raid rather than carry it with them as they know they are coming back the same way.
If the characters need to take ship at any point in their adventures it is a great time to introduce the orcs by having them come upon a town that has been treated to the orcs attention. From the landward side the characters discover the remains of burned farms and when they do reach the port they discover the wrackage made of the harbour. Everyone will be able to tell the characters that all this happened because the town council refused to pay the orcs their tribute. There will probably be people who blame the authorities and those that support that decision.
The orcs are not above a bit for piracy and in particular they like to take captives for ransom. thse plans often don’t work out too well as they have been known to eat the captives regardless of whether the ransom has been paid or not. They once managed to ransom an entire ship and its cargo back to the owner as it was to big for the orcs to sail and the cargo (grain) was no use to them at all.
A kidnapping is a great way to have the orcs and characters cross paths. It is simply the heroic thing to do to rescue the victim. You should bear in mind that the orcs have not survived this long because they are stupid. If the characters rock up and are obviously more powerful than the orcs then they will do whatever they can to distract the players long enough to make their get away. Things like dropping the victim over the side with heavy iron manacles in their feet so they disappear under the waves like a stone. Launchig fire arrows at the characters vessel to cause a distraction and positioning their boat so that they have the wind to make their escape whilst at the same time robbing the characters boat of the wind. If the orcs end up in a chase as they flee the characters the sorcerer will attempt a ritual to control the weather (a boat chase can last for many hours) and make good their escape that way.
What the orc will not do is make it obvious that they have a spell caster with them. Nor will thye all fight to the death in the first opportunity for battle. They are all greater orcs and they got that way by surviving many battles and schemes in the past and they want to again. Do not make the mistake of killing them off too soon as there is a lot of potential here either to be an enduring pain in the side of the characters but also as a part of the local culture and as use as a plot device for your characters.