Pictish Board Warriors are a unique unit in Stronghold 2 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, because they will get annihilated. Please also be aware that no AI Lord in the game recruits these units.
You can recruit Pictish Boat Warriors from the rank of Knight Errant or higher. All you need is 40 gold in order to purchase this Mercenary, along with 1 peasant from the campfire.