Enrique -
   For many years, Menemsha Pond has been officially closed to overnight anchoring by the town --- and I strongly doubt that you could navigate through the channel into it, anyway, even at high tide.    As a teenager, I was in there way back in 1969 on a family member's 30' boat, and it was dicey; and its now many years that the channel has not been dredged (indeed, the channel bouys marking the passage have been removed).  Even when we dingy in to explore the pond -- a fun trip! - we've noticed how shallow the passage is.
   Moorings at Menemsha harbor are available, however.   There are 2 moorings -- only for sailboats -- inside the tiny harbor, at which there will always be 3 boats rafted on each (i.e., a total of 6 slots available)   There are additional moorings just outside the harbor in the cove off the beach  -- we were on one just 3 nights ago (see photo).  One can also anchor outside, off the beach, as well, good holding in sand.
   The inside moorings are obviously more desirable -- the outer ones can get rolly (try springing the rode if it is) -- and tend to go quickly.  The harbormaster won't take reservations until the morning of the day you are planning to arrive.  The trick is to place a telephone call as soon as the harbormaster's office opens at 7:00 am (if you call at 7:01 am, the phone will likely already be busy with other candidates!).  
  I've never tried Tashmoo (long on the bucket list), but I've been told by local sailors that its do-able.  Downside is that its far from anything (but that can also be a good thing, of course).
   Enjoy!
- Matthew