The ornamental design for a submersiblemixer, as shown and described.
FIG. 1 is a front view of the submersible mixer showing our new design;
FIG. 2 is a rear view thereof;
FIG. 3 is a right side elevational view thereof;
FIG. 4 is a top view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is a front perspective view thereof;
FIG. 7 is a rear perspective view thereof; and,
FIG. 8 is a left side elevational view thereof.
Portions or features of the submersible mixer not shown in the drawings form no part of the claimed design. The broken lines in the drawings form no part of the claimed design.