As it turns out, the spraydeck is essential for running whitewater, otherwise the boat will swamp. An Alpacka full of water is no longer a seaworthy craft, and lacking a keel can be difficult to maneuver through rough water to shore in order to bilge.
Alpacka rafts can be a lot of fun, and can add a whole new dimension to the backcountry experience. They also possess the unfamiliar power to put backpackers onto obscure water where downstream conditions are a complete unknown. Please be careful out there. Use the Force, but use it wisely.