The ornamental design for the floating thermometer, as shown and described.
FIG. 1 is a perspective view of a new ornamental design for a floating thermometer.
FIG. 2 is a front elevational view thereof, the rear elevational view being a mirror image.
FIG. 3 is a right elevational view thereof.
FIG. 4 is left elevation view thereof.
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a bottom plan view thereof.
The broken line showing of structural features is included for the purpose of illustrating non-claimed subject matter and forms no part of the claimed design.