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