Feels like murder/suicide to me. Some circumstantial evidence of it might could also appear to suggest foul play - like bank accounts emptied for example or dog shot etc etc.
If it was murder-suicide and Brian was the driver, wouldn't Joni have at least unbuckled her seatbelt? I believe she had some intellectual disability, but would that stop her unbuckling? Or perhaps Brian drugged her, drove around until she fell asleep, and then drove into the water?
If it were an accident, wouldn't at least Brian's seatbelt be unbuckled? How likely is it for persons who crash into water are knocked unconscious immediately and unable to try to escape? How common is it for victims who died in this way to be found buckled up?
Both seat belts buckled is puzzling to me. If foul play is suspected, then I guess they think Joni and Brian were perhaps killed or drugged, then placed in car and car pushed into water? That seems pretty far-fetched, I realize... Just trying to work out the possibilities...