Question

How did boats spread the disease

Answer 1

boats spread disease like any other mode of transportation does.
for example: if one infected person from point A rides a boat to point B and hugs someone, 1 person from point B is now infected. vs. if the person from point A just stayed home instead of spreading infection from that boat ride.

Answer 2

When an infected flea bit a person, they got infected with the plague. Some forms of the plague were spread through the air, others were spread from bites. If you breathed in infected air, then you would catch the virus. People didn't have enough knowledge about the plague to know that the rats carried it.