The ornamental design for a flotationdevice, as shown and described.
FIG. 1 is a perspective view of a flotation device closed around the back of a hat;
FIG. 2 is a perspective view of the flotation device;
FIG. 3 is a top view of the flotation device;
FIG. 4 is a bottom view of the flotation device;
FIG. 5 is a front view of the flotation device;
FIG. 6 is a rear view of the flotation device;
FIG. 7 is a right side view of the flotation device; and,
FIG. 8 is a left side view of the flotation device.
The broken lines shown in FIGS. 2 to 4 represent portions of the flotation device that form no part of the claimed design. The broken lines shown in FIG. 1 merely illustrate an environment in which the flotation device can be used and form no part of the claimed design.