The ornamental design for, an enginecover of an outboard motor for a vessel, as shown and described.
FIG. 1 is a perspective view of An engine cover of an outboard motor for a vessel, showing my new design;
FIG. 2 is a front elevation view thereof;
FIG. 3 is a rear elevation view thereof;
FIG. 4 is a top plan view thereof;
FIG. 5 is a right side elevation view thereof; and,
FIG. 6 is a left side elevation view thereof.
The broken line showing of a portion of an engine exhaust treatment device housing is included for the purpose of illustrating environmental structure and forms no part of the claimed design.