FWIW, I never got up to the southern Nicaragua border but spent some land time on the east coast of Costa Rica from where the highway hits the coast and then down south to Cahuita. All I remember of the northern end of the area is that it was flat and swampy and very humid. The land and the scenery got much prettier as we drove south towards Limon. And there seemed to be a lot of offshore reefs in that area, which were very noticeable because the waves were 5 to 8 feet for a few days due to the remnants of a large T/S before we got there. Very interesting area, but I'd see if there are some current
charts available.
Good luck........