M_Boats: dc vs ac drive

It would seem to me that a A/C drive would be better than a DC drive trolling motor, because it has no brushes yet is easy to speed control, Can be sealed so never need to open to the weather and salt for the life of bearings. The DC brush-less design seem to be far too expensive to use. But can not find a A/C drive trolling motor, is it a manufacturing reason yet to be completed, an efficiency problem or some thing to do with A/C drive in salt water.