Does a small boat trailer need to be registered in WI? i had it in MN and got a ticket for no registration.?

borrowed the boat from my grandfather who lives in WI. the boats registered in WI but i got a ticket for improper registration well it was parked at the boat lunch and i was on the water with the boat. do you need to have a trailer for a small 14 ft boat registered in WI? if not i should just be able to prove its from WI then....