As the boat moved off very quickly across the water.我想问,为什么是

move只是“移动,搬迁,使行动,改变.位置”的意思；
move off 是强调“（从某处）离开,出发,开始移动”
在本句中,As the boat moved very quickly across the water：当这艘船迅速在水面移动”不强调方向,目的等.
而As the boat moved off very quickly across the water.“船迅速从水面驶过离开,走掉”之意.