The Pictish Boat Warrior is an exclusive unit in Stronghold Legends, and cannot be purchased directly in game by any faction. Therefore, you cannot have any Pictish Boat Warriors in any Custom Skirmish or Multiplayer map.
Note you also cannot directly place a Pictish Boat Warrior on a map from the start of the game through the game editor. In order to use a Pictish Boat Warrior in your game this must be done solely by using a scripted invasion within the game editor.
We have been told by Firefly that the reason behind this was due to the fact that Pictish Boat Warriors are only used in the first chapter of the King Arthur campaign, and therefore were never really taken into account in terms of the main design and game balancing.
Once killed, no more Pictish Boat Warriors can be recruited on the map and you would require another scripted invasion to replace any which are lost. They are however a unique unit in Stronghold Legends in that they are able to traverse water.
They are the only unit in the entire game with such capabilities. Once these units reach water they will automatically use their shield as a boat, and use their Labrys (weapon) as a oar to paddle with.
With a lack of any Armour and relatively slow movement, their only real use is for ambushes through water or to coordinate other attacks. They are only really best used in larger numbers, because they get killed very easily. Keep these units out of range of all missile units and Men-at-Arms, because they will get annihilated.
The Pictish Boat Warrior also cannot defend himself against any aerial attacks of any kind. Note that if attacked by a Giant Demonic Bat and thrust into water or sea, despite his ability to paddle through water the Pictish Boat Warrior will still be killed. Pictish Boat Warriors can also be frozen by units such as the Ice Queen.