The trailer is quite narrow and fits between the Pontoons. The Trailer actually lifts the deck of the Boat and does not support the Pontoons and the wheel base is quite small. This style of pontoon boat trailer can be retrieved and launched in the shallowest of water, since the Pontoons are very low to the ground.
Tanking is, as its name suggest a method of sealing a wall to prevent water ingress (think 'water tank). often mis-specified as a cure for damp problems it is basically a waterproof cementious slurry (wet mix) that is applied to a surface to prevent water penetrating the internal surface of a building..
Temporarily layout the new wall. Run a stud finder along the ceiling to locate the joists. If the joists are parallel to the new wall, adjust the layout so that the wall is directly under a joist. If they run perpendicular to the wall, put the wall where you want.