I have crossed the Mona Passage
6 times in 7 years and on four of them it was flat calm or seas under 3 feet. That was because I was willing to wait in Samana for the few days to a few weeks for the perfect alignment of weather
fronts off Florida
to block the normal trades and flatten the seas.
To avoid the "confused seas" avoid the "Hourglass Shoal". Van Sant discusses the best crossing techniques in his books
. But basically I head
east-southeast from Samana until Punta Nisibon and then straight to N18 - 41' W067 - 55' to avoid the Hourglass shoals. From there direct to Mayaguez or Boqueron. The point is to stay north of the Hourglass Shoals to avoid the huge "upwelling" of ocean water
when it hits the north wall of the Shoals and makes standing waves and confused seas. When the massive currents flowing along the north coast of P.R. bend into the Mona Passage
is forced from 3000 meters to 80 meters by the north wall of the hourglass shoal. It is not a pleasant place to be so just head
to a waypoint reasonably north and then turn southeast towards Mayaguez. Also a night passage if best as outlined in Van Sant's book as you avoid the T-storms that roll west off P.R. and also you arrive in P.R. in the early morning which is quite convenient for checking in and getting settled.