Sonotube or blocks for shed base
Sonotubes do not heave because of groundwater, they heave because of frost. Whether or not you get heave is a function of soil type, winter temperature, and soil moisture. Basically silty soil is prone to heave, whereas coarse sand and gravel does not heave, and neither does clay. Unfortunately most people can't tell the difference between clay and silt, so folks often mistake one for the other, and therefore the urban myth that clay is prone to frost heave has developed.
It is also relatively simple to place concrete even in standing water, the technique has been discussed repeatedly on this forum, do a search and you will find numerous threads on that topic.
So you can go either way, block or concrete in a sonotube. Course you need a permit, and being so close to a wetland you are going to check first, build later. But once you have your approvals, either technique will work. If the shed is going to house valuable stuff like a wood shop, I would carry the sonotubes down below frost level, which is about four feet in MA. If you are not putting valuable stuff in the shed, two feet should be fine, and if it settles or heaves a little, no worries.