Pjard,
Ball park and for planning purposes, it's line of sight.... but that's radio line of sight, it doesn't mean that you can actually see, meaning, nothing in your way, type of line of site.
As an example, this weekend, I was at the docks at Meridith, and I heard someone talking from the West Alton Marine area. That's almost from one end of the lake to another.
As stated, the type of antenna being used on both ends, as well as the height of the two antenna's can make a great difference. Also the quality of the installation (type and length of cables used, etc.
Also, sometimes atmospheric and other conditions can occur to cause signals to travel further or shorter than you expect. Some signals can bounce off of things, but mostly this doesn't happen very often with VHF marine frequencies.
As an example, sometimes, when I'm on the south end of the broads, about by Rattlesnake Island, I sometimes can weakly hear a Coast Guard station somewhere on the coast. I can only guess that they have that antenna quite high on a tower.