The ornamental design for a buoyantplatform, as shown and described.
FIG. 1 is a top front perspective view of a buoyant platform;
FIG. 2 is a top plan view thereof, the bottom plan view being a mirror image thereof;
FIG. 3 is a front elevation view thereof;
FIG. 4 is a rear elevation view thereof;
FIG. 5 is a right elevation view thereof, the left elevation view being a mirror image thereof; and,
FIG. 6 is a top front perspective view of the buoyant platform, shown in use.
The broken lines showing a user of the buoyant platform in FIG. 6 depicts environmental subject matter that forms no part of the claimed design, while the remaining broken lines shown in the drawings depict portions of the buoyant platform that form no part of the claimed design.