Short answer: It's the dynamic pressure which is at work in a sea anchor:
Let's see how we can separate effects of static and dynamic pressures.
Use the water similarity:
- Blow up a balloon and submerge it in water.
- As you go deeper, the balloon volume shrinks as the static (ambient) pressure increases.
- Move the balloon at some speed through the water, its shape changes because the dynamic pressure created by the displacement is not equally distributed over its surface.
Dynamic pressure is comparable to static pressure unevenly distributed. It's dynamic pressure that slows down a parachute. If there was no static pressure, the effect would not change.
To observe the effect of dynamic pressure alone, you can replace air by water in the parachute, this is what is at work in a sea anchor.
(You can also demonstrate this in air, but this would requires to inject air under a parachute in vacuum, not simple...)
To be complete: In this experience the balloon also receives a buoyancy force because air and water densities are not equal. A the weight is added to stabilize the balloon. When the balloon contains water, this buoyancy force disappears and the weight is not required any longer.