Vanilla cars and trains don't have equipment grids - only tanks and spidertrons. Since cargo ships fit more alongside the car/train side of things, I'm not sure they should gain one. Plus: equipment grids are quite fiddly to set up on vehicle, and there's already enough fiddlyness with setting up cargo ships as it is. And there's enough scripting around cargo ship placement/blueprinting/removal that this may not be a trivial feature to add.
