you can't do much, but a couple things to can do is stay on there ass, call them every day or even several times a day, and find out what is going on, make the investigators, know you, your father, your family, by sight, name, voice, etc. and hopefully care about yaw, if not be annoyed by the constant questioning, to make them want to help you, if not because they care, but at least to make you stop calling, or coming by, or whatever. and a other thing you can do, is go to the news media, and make a big deal about the way things are being handled, but you have to do this in a diplomatic way to keep yourself from being targeted, by the killers, or the police, if you piss them off.
I am sorry for your loss. I know not having answers is hard, but from what I understand bodies that have been in water are harder to autopsy. I wish there was a kinder way to say that. Investigations take time and while they are going on the police really can't tell you much. I know you want answers and I hope you get them soon.
I'm sorry to hear this. Unless you have the funds to hire a private investigator all you can do is email or call the lead investigator once a week and ask for an update. At the 6 month or year mark (hopefully you will get news sooner) you could reach out to the media or "America's Most Wanted" if that show is still on.
hire a private investigator they will do all they can to review evidence and see what can be done to get the facts and do something. the police get paid weather they work or not. so there isn't much interest in working your case unless it falls into their laps
Unfortunately if you bug the police, then you are taking time away from them finding their answers. I know it is frustrating waiting, but these things do take their own timeframe and not ours. If after a month more you still don't have any more answers, then I would make myself heard. They need the time to follow up any clues they have. I am truly sorry for your loss.