Let me begin this thread with my own suggestions:

1
Small boat types to be hanged on those typical boat holders or stored on the deck of big rafts.
2
Add the left/right OARs. Players can change oar angle/length. Paintable lower/upper parts of an oar. The oars, when mounted to any wall, will move accordingly to the raft direction. Oars snap to pre-defined positions on a wall, up to 3 lines of oars above each other, without reaching the upper/lower wall edge. Each higher line is slightly moved towards the aft of the raft. I can give you a sketch. I am active in the field of maritime archaeology, and reconstruct ancient Roman ships. Let's create some Roman galleys here on Ark. :-D
3
Add a paddlewheel! Mounted to the raft side/end (front). Diameter/width adjustable by player. Rotation accordingly to raft's moving direction. Paintable. Side ends of their middle axis mountable to walls or floors (ceilings).
4
I can give you sketches of many more historical/contemporary types of sails. Would you be interested?