A float plan is a written record indicating the planned destination and approximate length of time for the outing.
Sample forms are available at the Coast Guard's website here: http://floatplancentral.cgaux.org/
One should be completed and left with a relative or friend prior to each trip. In case of an emergency or failure to return within a reasonable period of time, pertinent information will be available to assist local marine police or the Coast Guard in determining whether a search should be performed.
Be sure to notify the float plan holder upon return.