The ornamental design for a flotationdevice, as shown and described.
FIG. 1 is a perspective view of a flotation device in accordance with the invention, a strap therethrough illustrative of an environment for the floatation device not forming a part of the design and therefore being shown in phantom lines;
FIG. 2 is a left side view perspective of the flotation device in FIG. 1, the right side being a mirror image of the left side;
FIG. 3 is a second left side perspective view of the flotation device;
FIG. 4 is a top plan view of the flotation device; and,
FIG. 5 is a rear elevational view of the flotation device.
The bottom view of this design is not being claimed for any ornamental purpose, and so is not included herein.