Self-propelled boat movers are not trailers in the strictest sense of the word. They are hydraulically operated boat movers, with their own Prime mover (tractor unit). They share all of the features of hydraulic boat trailers.
Glidepath, also known as a "Float-on" style trailer allows the boat to float onto the trailer; after trailer has been partially submerged (usually 3/4 of the trailer). Since its inception it has become quite popular v.s. the "Roller style trailer".