KBOS has eight major runway ends:1 04L, 04R, 09, 15R, 22L, 22R, 27, and 33L.
As KBOS is primarily situated on (and occupies the vast majority of) a peninsula made by dumping lots of sand and dirt into Boston Harbor, and since making new land (especially new land that's going to be left completely unused except on the rare occasions that an aircraft goes off the end of a runway) is expensive, only three of said eight major runway ends (22R and 27, which stop well short of the edge of the land,2 and 33L, which runs back up along the neck of the peninsula) are followed by full-size runway safety areas (RSAs):
KBOS's other major runway ends (04L, 04R, 09, 15R, and 22L) drop off into the ocean just past the end of the runway. As running off the end of a runway into the ocean is generally bad for non-seaplane aircraft, this lack of RSA space would generally (in the U.S., at least) call for the installation of EMAS beds to catch aircraft overrunning these runway ends. However, only two of the five aforementioned runway ends (04L and 15R) are actually equipped with EMAS; runways 04R, 09, and 22L each have only a short, un-EMASed blast pad separating the runway end from the dropoff into the harbour.
This is especially worrying when one considers that 04R and 22L are two of KBOS's five primary instrument runways (and 04R is one of the airport's two ILS-III runways, the other being 33L) - i.e., two of the runways that the airport preferentially uses in the very weather conditions that lend themselves to runway overruns!
Why are only two of the five water-facing runway ends at KBOS equipped with EMAS beds to keep them from discharging aircraft into the sea?
1: All references here to "runway ends" refer specifically to the departure end of the runway(s) in question, since that's the end that aircraft tend to run off of.
2: And whose RSAs cross over each other not far past the ends of their respective runways, as seen here.