I claim the ornamental design for a pontoon boat wall, as shown and described.
FIG. 1 is a perspective view of an exemplary pontoon boat including the pontoon boat wall;
FIG. 2 is a top view of the pontoon boat wall;
FIG. 3 is a right side view of the pontoon boat wall;
FIG. 4 is a left side view of the trim piece;
FIG. 5 is a rear side view of the trim piece; and,
FIG. 6 is a front view of the trim piece.
The broken lines shown are included for the purpose of illustrating an exemplary environment in which the article resides and form no part of the claimed design.