The ornamental design for a fillingdevice for waterballoons, as shown and described.
FIG. 1 is a front view of the filling device for water balloons showing my new design;
FIG. 2 is a left side thereof;
FIG. 3 is a top plan view thereof;
FIG. 4 is a bottom plan view thereof;
FIG. 5 is a top perspective view thereof; and,
FIG. 6 is a bottom perspective view thereof.
The broken lines shown in the figures are for the purpose of illustrating portions of the filling device for water balloons, and form no part of the claimed design.