Two activists who helped the migrants from Afghanistan, Sudan and Syria board the vessel have also been convicted.
They were among a group of people who broke through a barrier at a protest to board the Spirit of Britain ferry.
It comes as local authorities prepare to evict migrants from part of the camp in Calais known as the Jungle.
About 1,000 migrants in the southern half of the camp have until 1900 GMT on Tuesday to leave.
The BBC's Lucy Williamson says that none of those sentenced to imprisonment by the court in Boulogne will be spending Monday night in jail because they have all served their sentences while waiting for their trial in custody.
Fines of â‚¬300 (Â£233) issued against the two activists have been suspended, our correspondent adds.
Six migrants have been sentenced to a month in prison for occupying a ferry in France last month in protest against Britain's asylum policies.