The ornamental design for a bellows for marine exhaustsystem, as shown and described.
FIG. 1 is a perspective view of one of the bellows of a marine exhaust system illustrating my design;
FIG. 2 is a front view thereof; all other side views being identical; and,
FIG. 3 is a top view thereof; the bottom view being identical.
Any elements shown in dash-dot lines form no part of the claimed invention, and are instead included to show environmental aspects of the claimed invention. The dashed lines form an unclaimed boundary of the design.