The only risk I can think of is one which you get in the ocean aswell, but seems to be more of an issue in bodies fresh water do to their confined space, and that is a lack of visibility. Living in Redding, CA, we have perfect examples of this. Lake Shasta is a nightmare to dive with depths of up to 500', standing submerged trees all around with lines stretched between them, boats constantly wizzing around overhead, and visibility rarely getting better than 4 feet while sometimes getting to absolutely zero if enough boats are out on the lake. Not to mention that if your going to dive in Shasta, you had better be prepared to meet some of the worlds largest sturgeon, catfish, and carp. Divers a few years back were sent to remove an obstruction from an intake to the dam (shasta dam is HUGE by the way) and when they approached what appeared to be a "...very large log" by the (at the time shut off) intake, it literally swam off and was later determined to be a sturgeon of undetermined size. Thats why I wont dive in shasta, and that is why most all divers from around here either go to Whiskeytown (not a clay bottom like shasta = better vis) where visibility is around 10-15', or out to the coast where its about the same or better. But even then, Whiskeytown Lake can have some terrible visibility if enough boats are out, or if a diver gets to stirring up the good 6'' of silt that sits on the bottom (you do that and its so fine, vis is ruined for the day in that area). So my conclusion would be that while visibility can be just as bad at times in the open ocean, it will probably have a higher chance of something going wrong in a confined body of fresh water by method of another man made object/danger i.e lines to be caught in etc. However, that being said, diving in the ocean is still considerably more dangerous imo.
Just my thoughts.