The ornamental design for a pontoonboat, as shown and described.
FIG. 1 is a perspective view of a pontoon boat showing an embodiment of my new design, a plurality of straps, a plurality of D-rings, and an arch being shown in phantom and forming no part of my claimed design;
FIG. 2 is a left side elevational view thereof, a right side elevational view being a mirror image thereof;
FIG. 3 is front elevational view thereof;
FIG. 4 is a rear elevational view thereof;
FIG. 5 is a top plan view thereof;
FIG. 6 is a bottom plan view thereof;
FIG. 7 is a perspective view thereof, a personal watercraft being shown in phantom and forming no part of my claimed design; and,
FIG. 8 is a perspective view thereof with drop-in flooring.