Few things, differences:
- car - thin body, might be easier for radio waves to penetrate, receiver mounted on top of the chases which is as good as it can get, distance RX/TX can be typically small.
- boat - hull can be thick, harder for the radio waves to penetrate especially if the antenna starts inside the RX as that is the case for some surface RX. Distance Rx/Tx can be more than with cars.
If you look at the marine RX you would see that the antenna dosn't start right of the PCB board but instead a cox cable is connected to the PCB. It is hard do see for the pic as I do not own one but the last few inches of the coax must have the shilled removed. That is the actual antenna that can be mounted high above the hull surface.
In my case the Rx is on the back of the hull that gets "under water" and with the splashing water around it, it is probably not helping.
I think if the surface RX is modified such that the antenna is removed and replaced with a coax cable that should fix the range issue.