It is legal in Wisconsin...
Quote from WI. boating regulatio website...
"Display of Identification Number and Decals: Upon being issued certificate of number and decal, the owner of a boat shall display the issued number on each side of the forward half of the boat. Once the certificate of number is received, the owner of the boat must purchase adhesive number and letters or paint the numbers and letters on the boat using block letters not less than 3" high and in a contrasting color to the surface on which they are applied. Display the registration number with at least a 2" space or hyphen between the letters and center 4 numbers. It is the responsibility of the owner to be sure that the registration number is clearly visible and maintained in legible condition at all times. Place the registration decal (indicates the date through which the registration is valid) in line with and 3" from the registration number, toward the stern. In Wisconsin, registration expires on March 31 of the year shown on the decal."
Quote from actual "Wisconsin Boating Regulations" booklet your receive when registering a boat in your name...
"The Certificate of Number (registration card) must be on board
and available for inspection by an enforcement officer whenever
the vessel is operated. The registration number and decals
must be displayed as follows:
• Numbers must be placed on each side of the forward half of the vessel
• Number must be read from left to right
• Numbers must be at least 3” high in BLOCK bold letters
• Numbers color must contrast with the background (boat) color
• Letters must be separated from the numbers by a space at least
two inches wide
• Decals must be affixed 3 inches behind (toward the stern)
and in line with the numbers
• No other decals or numbers may be displayed on the
forward half of the boat."
Outdoorsman