The ornamental design for hydration bladder, as shown and described.
FIG. 1 is a front perspective view of a hydration bladder illustrating our new design in an unfilled state;
FIG. 2 is a front view thereof;
FIG. 3 is a back view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof;
FIG. 8 is a front perspective view of the hydration bladder illustrating our new design in a filled state;
FIG. 9 is a front view thereof;
FIG. 10 is a back view thereof;
FIG. 11 is a left side view thereof;
FIG. 12 is a right side view thereof;
FIG. 13 is a top view thereof; and,
FIG. 14 is a bottom view thereof.
The broken lines in the drawing views showing portions of the hydration bladder are for the purpose of illustrating environmental structure and form no part of the claimed design.