While it is true in MA that the land owner does own to the low tide mark, the public has an easement for fishing, fowling, or navigation. This includes anything between the low and high tide marks. The courts have upheld this multiple times. So, as long as your feet are touching water, and you are actively fishing, there is no way for you to be tresspassing. It is a protected right.
This does not, however, include the right to cross private land to get to the shore. You must get public access, and then you are free to walk as many miles as you would like.
I have since printed out and laminated the below text. I encourage you to be respectful, not draw attention to yourself, and pick up any and all garbage. However, as a fisherman, it is your right to be there- regardless what the (often) entitled land owner says.
Public Access to the Ocean and Private Property
Private property in Massachusetts is typically owned to the mean low water (MLW) mark, and the Commonwealth of Massachusetts owns lands below the MLW mark. However, Chapter 91 of the Massachusetts General Laws, preserves the public's right to "fish, fowl, and navigate" waters of the Commonwealth, including lands in the intertidal zone (between the high and mean low water marks), even if it is privately owned. This law does not convey the right to cross private property or uplands to reach these areas. Some Chapter 91 licenses provide for public access to intertidal areas across intertidal areas.
MGL Chapter 91 Waterways: Section 1 maintains the historical right of access to FISH or FOWL, defining Private Tidelands as “tidelands held by a private party subject to an easement of the public for the purposes of NAVIGATION AND FREE FISHING AND FOWLING and of passing freely over and through the water”.
Right to fish includes right to seek or take any fish, shellfish, or floating marine plants, from a vessel or on foot.