If the tanker acclerates in the positive x direction, then the water will bulge at the back of the tanker. The free surface will be such that the tangential force on any fluid parcel is zero.
Consider a parcel at the surface, of unit volume. The forces on the fluid are
-rgŷ and -rax̄
The component of the weight along the surface is rg sin θ
The component of the acceleration force along the surface is
r a cos θ
rg sin θ = ra cosθ
Hence, tan θ =a/g