I know 1 body was dumped by boat or at least I suspect it was. SS has publicly said he doesn't believe either of them were killed at the house but I disagree and here's why. There is too much risk to bring them outside of the house . Lets say the Killers arrived by boat and they took the Dermonds from the house, blindfolded, wrists tied up, gagged so they wouldn't scream etc. Now picture the killers driving a boat on the lake with 2 elderly people onboard tied up, What if a fisherman sees them? It's possible because even if this happened at night ( which fisherman love fishing at night) there's a risk of being seen. Now they obviously brought Mr Dermond back to the house either by boat or by car to behead him in the garage (after he was killed somewhere else) Another risky thing to do. So after the beheading they left the residence yet again by car or by boat that's a lot of activity in less than 24 hours and probably less than a few hours. No matter what time of day or night they came and went they took great risk in leaving and coming back. For a quiet lake things like this make people notice. I'm thinking they were killed in the house or garage. Not sure if there is a curfew or not for having the boat out on the lake maybe there is if so this probably didn't happen at night or they didn't come by boat.
Just typing my scambled thoughts as they come to mind