The ornamental design for an inflatabledevice, as shown and described.
FIG. 1 is a top, front, right-side perspective view of an inflatable device;
FIG. 2 is a back, bottom, left-side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a back view thereof;
FIG. 5 is a left-side view thereof;
FIG. 6 is a right-side view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The longer dashed broken lines depict portions of the inflatable device that form no part of the claimed design. The shorter dashed broken lines depict a fringe trim material of the inflatable device that forms no part of the claimed design.