The ornamental design for a bracket for attaching a thruster to a boat, as shown and described.
FIG. 1 is a perspective view of the bracket for attaching a thruster to a boat of the present invention.
FIG. 2 is a front elevational view.
FIG. 3 is a back elevational view.
FIG. 4 is a right side elevational view.
FIG. 5 is a left side elevational view.
FIG. 6 is a top plan view; and,
FIG. 7 is a bottom plan view.
The broken line showing is included for the purpose of illustrating portions of the bracket for attaching a thruster to a boat and forms no part of the claimed design.