I have seen similar proposals to deal with potential sea level rises in New York City, which would work by constructing levees around the east river and the barrier islands surrounding Staten and Long islands. As Boston has a system of barrier islands, I imagine a similar project would be feasible, but I wonder how realistic it is logistically and financially speaking. What would be the relative pros and cons of such a proposal?
The key thing to understand is that the first effects of a rising sea level would be more flooding events during nor'easters and then more during exceptional high tides and then during ordinary high tide and only finally if the sea continued to rise, all the time.
London's Thames barrier (which is really quite neat) is designed to be closed only when there's a storm which pushes a high tide upriver -- it's closed a few times a year exception in a couple exceptional years. The proposal for NYC is a much larger affair, but basically the same idea, and would perhaps buy fifty years even if the sea continues to rise.
Boston could undoubtedly do the same, but it's always a cost-benefit tradeoff and might not be worth doing. For example, it may prove cheaper to raise the city by requiring new buildings to be 50' higher. (Or perhaps they could buy hip boots for everyone.)
In the end, it depends on how much sea level rise they want to plan against, since the more extreme scenarios would have the coast flooding in many places and they would all need to be defended or the sea could just go around a barrier in Boston Harbor.