The sidebar scores display the entities' Motion[0] value by working always command
"execute as @e[type=boat] store result score @s motion_x run data get entity @s Motion[0] 1000" 

"motion_x " is a scoreboard here is the gen-command for it.
"scoreboard objectives add motion_x dummy "实体动量x""

At first, I thought it was my command's fault. But when I installed the mod to dispaly the speed of the boat, I tried to use data get command when the boat was sliding, but the value was still 0

When I left the boat and push it to slide,the Motions' values are normal.

Even if I try to use "/ data get entity @s RootVehicle.Entity. Motion" to get players to ride the motion of the entities is alwyas 0