LDP,
To the contrary, “motor vehicle” does include motorcycles. The Texas Penal Code § 32.34(a)(2) defines motor vehicle as: “a device in, on, or by which a person or property is or may be transported or drawn on a highway, except a device used exclusively on stationary rails or tracks.” This definition uses the term “on” in reference to motor vehicles. This definition is extremely broad and does in fact include motorcycles. Similarly, watercraft is defined very broadly, and would include jet skis. Under Texas Penal Code § 46.02(a)(1), if you are not an LTC holder, your handgun must be kept out of plain view. Typically, motorcycles and jet skis, alike, contain a compartment either in front of, behind, or under the seat where you can conceal your handgun from plain view in compliance with the statue. If you have an LTC then you can openly carry your handgun in a shoulder or belt holster, conceal it on your person, or store it you your saddle bags.
To address your second question, traveling “en route,” literally means to travel directly to a motor vehicle or watercraft. Under this section, a non-licensed person could carry a concealed handgun from their vehicle in the marina parking lot directly to their boat. However, the more you stray from a direct route, the less likely you will be considered “en route.” If you were to stop and get a sandwich before getting in to their boat, you would no longer be considered “en route” under the statue.