a technical explanation is preferred

probably because a boat has more drag, being the water. holding it back, and the more drag there is, the less fuel economy you wil get

water is thicker than air

Definitely the drag of the water on the hull causes friction that is much greater than the friction of the wheel bearings on a vehicle. The water is friction is also why you do not need brakes on a boat.

As alluded to, take a look at the kinematic viscosity of water versus air. Then calculate the amount of area in the water & in the air for a typical boat.

Roughly, & 20ft bass style boat requires 200hp at 70mph due to the higher drag compared with a road vehicle. A full size pick-up requires less than 50 hp at 70mph. In addition, boat engines, especially outboards, r not as efficient as automotive engines. Outboards r primarily designed for power & light weight, then environmental regulations, with fuel economy bringing up the rear.

because you have to constintly give it gas to propell your boat you stop giving it gas and what happens it stops
a car rolls even when you stop giving it gas

ftyzgzfzxgfxgtf

Imagine driving your car uphill 100% of the time, in low gear at 5500 RPMs. It'd get pretty poor fuel economy.

Boat motor has to lift and hold the hull out of the water. Never coasting, or loafing along at 1200 RPMs.

you are revving at 6500 rpm, so it is using gas.

More drag
Boats never coast
Heavier loads
Run at higher RPM's

Okay so now you've been told about drag & friction.

All minor components of the answer u need.

Another minor point is the tuning of the engine & fuel burn rate to deliver a given horsepower. Given size restrictions of a marine engine application.you need the most bang for the size available.

but here is THE Real deal...

Wheels put the power directly to the road.assuming no spinning & an efficient transmission & efficient throttle application. You have a very high rate of power transfer due to friction(tires+road).

You do not get this in a boat.it takes a while for a prop to bite.even jet drives have wasted wash. Marine power delivery is very inefficient. Also props have different pitch(blade angle) & may be missmatched to the drive unit or hull. You have a very low rate of power transfer to to lack of friction or more particularly.slippage within the media(prop+water).

Simply put most road based vehicles r hold to hold by the bumper when they're in gear. It is not hard to hold a boat to the dock even when it is in gear.