The ornamental design for a sidelamp for a boat, as shown and described.
FIG. 1 is a perspective view of a sidelamp for a boat, showing my new design, with the wire broken away to indicated indeterminate length;
FIG. 2 is a front elevation view of the sidelamp;
FIG. 3 is a rear elevation view of the sidelamp;
FIG. 4 is a right-side elevation view of the sidelamp;
FIG. 5 is a left-side elevation view of the sidelamp;
FIG. 6 is a top plan view of the sidelamp;
FIG. 7 is a bottom plan view of the sidelamp;
FIG. 8 is a perspective view of the sidelamp mounted on the left side of the boat; and,
FIG. 9 is a perspective view of the sidelamp mounted on the right side of the boat.
The broken line showing of structural features is included for the purpose of illustrating non-claim subject matter and forms no part of the claimed design.