Yes, Dennis should have spilled out EVERYTHING for the detectives, but instead he listened to RJ and gave his statement through an attorney Wagner hired for him. Because he DIDN'T see exactly how Natalie got into the water, he thought it best to just follow instructions and not to cause more trouble. Dennis once said that he really believed the detectives would figure everything out anyway, and no one was more astounded than Dennis when that didn't happen. By that time he was holed up in a Wagner house bedroom and scared to death to make a move without checking with RJ first.
Dennis's family and friends and his then fiance wanted him OUT of that house. We all saw the transformation in Dennis as it occured. Dennis stayed loyal to RJ until he realized that what haunted him was also killing him. Although he didn't see how Natalie got into the water, he did see and hear enough to know WHY she ended up in the water. There was NO banging dinghy. Dennis was a witness and his account passed on a polygraph test. If that isn't an honest attempt to prove who's telling truth and who's telling lies, I don't know what is. Notwithstanding, his account is the only sensible explanation ever offered. His account meshes with every documented fact and known detail of that tragic night. It's rather compelling when you think about it. Good detectives usually know that when everything finally meshes, and FITS, therein lies the answer to any mystery.