There's not a HUGE gap between services. You've got 2190 at 6:20, 190 at 6:55, 2150 at 8:03, and 170 at 8:30. You don't get hourly service to Boston like you do to DC, but you still have fairly frequent service considering the constraints placed on the NEC by the Coast Guard.
I was writing about the gap in the early AM trains from NYP to BOS compared to the southern half of the NEC. For southbound departures from NYP to PHL and WAS on weekdays, after #67 departs at 3 AM, there are Regionals departing at 4:40 AM, 5:30 AM, 6:05 AM; Acelas departing at 6 AM and 7 AM. I expect the #151 Regional departing NYP at 4:40 AM does not get a lot of passengers out of NYP, but is mainly for the PHL, WIL, BAL to WAS market.
For northbound departures from WAS to NYP on weekdays, there are Regionals at 3:15 AM, 4:00 AM, 4:52 AM, 5:30 AM, 6:30 AM; Acelas at 5 AM, 6 AM, 7 AM. The really early Regionals leaving before 5 AM presumably don't depart WAS with many passengers, but get WIL, PHL to NYP and for the 2 that run through NYP, NYP to BOS markets. I took the 5 AM Acela from WAS to NYP every couple of weeks s few years ago for business trips; typically left WAS with maybe 50-80 passengers, was full when it departed from PHL to NYP.
In comparson northbound from NYP, after #66 departs at 2:40 AM, the Acelas depart at 6:20 AM, 8:03 AM; Regionals at 6:55 AM, 8:30 AM. If there were slots available and enough Acela trainsets (and a faster ~3:15 trip time would help), it is a good bet that there would be an earlier 5 to 5:30 AM Acela from NYP to BOS, scheduled to arrive at BOS before 9 AM for the business market.