The flag of the Navy of Massachusetts is a variation of the Pine Tree flag flown by six cruisers under command of George Washington in 1775.

The Pine Tree was a symbol of liberty; probably from the pine trees that grew in abundance in what is now the State of Maine, while it was then still a part of the territory of the Massachussetts Bay Colony.

Significance of the rattlesnake as a symbol is provided under description of the 'Culpepper, c. 1776' flag. That description is repeated here to avoid going back to that flag description:

The rattlesnake symbolized that the American Revolution was like the behavior of a rattlesnake - it did not attack unless it was provoked.

Thirteen rattles symbolized necessity of the thirteen colonies to make a noise only if they were united.