No. It generally is not. Remember that when a boat with any kind of motor goes down there will be "environmental damage" i.e. fuel, motor oil, antifreeze, battery acid, transmission fluid. All kinds of nasty stuff. From what I read, liability insurance only covers damage to "property" which would, of course, exclude the environment. From what I've heard, when a boat sinks near a populated area, the EPA almost always gets called in and thats where the fun starts.
AND you will be held liable for the cost of removing the wreck if it is in a place that affects boating traffic.
Bottom line is that the value of the boat may be the LEAST of your losses.