Obviously it's the fish eye lens effect. The light reflects off all the fish eyes in the water between boston and the cape, causing this distortion and making us think the surface of the planet has a curve. The more fish out swimming the more distortion and therefore curve.

The easiest way to see this will be when global warming and ocean acidification (which doesn't exist by the way) causes a mass extinction in the oceans, fully depleting them of fish life. At this point the effect will be gone and you will see the full height of the buildings. We can already see this happening due to overfishing (which also doesn't exist) causing you to be able to see the buildings today where as 500 years ago, when more fish eyes were in the water, the tops of the buildings were not visible from the cape.

/s