I'm looking for a late 1980's or early 1990's movie where the husband and his mistress plan to kill the wife and live together. It was an USA made movie and probably first or second movie of the series.
It was a low budget movie. There wasn't any well-known actors or actresses and in fact there weren't many people in the whole movie. There was a married middle aged couple. I don't remember that they had any kids. The husband is about 35-40, medium sized, blonde and had hairstyle like Leonardo Dicaprio's in Titanic. His wife was 35-40, had long hair, short about 5'4 or 1.63m , dark eyes, white skin and medium weighted.
The husband had a mistress but I couldn't remember her appearance. Probably younger and more attractive than his wife. They planned to kill the wife, because the husband thought that she wouldn't want to divorce and even if they got divorced, she wouldn't stop following them. They didn't think to kill her with any weapon or tool. They poisoned her to kill her without any evidence.
There was a funeral scene and after that the couple took a vacation on the husband's boat on the ocean. When they were on the boat, there was a scene of rainy weather at the cemetery. The wife, who everybody thought dead, opened her eyes and tried to get out of the grave. She hadn't actually died. The poison that they gave her had slowed the rhythm of her heart, make her had a deep sleep and made very hard to notice her pulse by the paramedics/doctors. There was a similar scene in one of the episodes of the TV show 'Shameless'. The kids of Frank Gallagher gave him horse calmer and it showed him seem like he was dead.
After that, the movie continued on the boat. The couple were happy and sailing on the ocean. Suddenly the wife showed up on the boat. But the couple didn't see her as she was hiding from them. The mistress went to the kitchen and the wife knocked her out without much noise. She tied her up and kept waiting for the husband. The husband got curious after a while and also went to kitchen. I couldn't remember if he saw his wife or not but the wife also hit him and he passed out.
There was a small separate cabin in the boat that allowed people do deep-sea exploration without any diving equipment. It exists in boats designed for scientific research. The wife locked them in that cabin, sent them to the sea depths and cut the connection between the cabin and the boat. They were sinking in the deeper parts of the sea and after a short time they disappear.
I'm sure it was a series and there were at least 2 movies in the series.