Much of the power to move a vehicle is in the startup and shutdown (acceleration from standstill then slowing down). Once trucks get going they don’t need as much power, but that startup takes a TON of energy. Ships on the other hand can get moving a lot easier, even if it takes more energy to keep them moving. They don’t have to do 0 to 60mph in 10 seconds to keep up with traffic. They can go 0 to 20mph in 20 minutes if they want. They also don’t have to slow down for stop signs and then speed up again. The drag helps them slow down, too. Finally, ships go a lot slower, so there is lower wind resistance to deal with (vehicles going 55mph spend something like 50% of their power to overcome air drag).