Young couple goes on van tour of US, boy comes back with van - no girl

I don't think there's a single answer for those questions - it's all situational. Seems to me that a boat at a dock or in a marina is likely subject to some kind of surveillance camera and would require keys or some sort or a highly technical knowledge of how to start a boat without keys.

A smaller boat with a manual start would likely be trailered somewhere and not simply sitting in the water ready to steal. So it would need to be put in, without being observed in the act of theft.

And then what? Drive a stolen, unknown boat to Cuba? I'm sure it's do-able weather permitting but that's not like hopping in a car and driving and I don't know of any evidence of him having significant maritime experience. Then, of course, what does he do in Cuba? I don't know but he would seem pretty conspicuous arriving in Cuba.
Surveillance cameras are easily defeated. Depending on the type of cameras it can be as easy cutting a cord with scissors then stealing the hard drive. If it is cloud based then you just hack into the system and delete the files too. If the camera system is monitoed on a network you just hack into the network then create a loop feedback of old video while the heist takes place. Sure, there are more complex systems out there but they too can be defeated using EMP's or infrared bursts,

For the smaller boat, it's quite easy to put in the water. There are typically three ways to do this. Simply steal a truck using either a flat head screwdriver, hotwiring the vehicle or if those fail simply use the keys that are resting above the visor. From that point just back the boat in and go.

Once you are in the boat, you just have to change your looks with something as easy as a hair cut or growing a beard. The most important part though is putting on a pair of Costas and wearing a Tommy Bahama shirt. At this point nobody recognizes you at any of your stops on the way to a tropical paradise. It also helps if you have Jimmy Buffet or reggae playing.