You'll need something like the following to be truly effective in removing salt, water by itself, even under pressure, won't really do it.This thread got me thinking... I checked to see what other options might be available for washing the underbody, and found these items on Amazon. I still need to look at them more carefully, but it looks promising from what I can see so far!
Buy Eastwood Road Film Paint Metal Protective Neutralizer 1 Gallon: Top Coats - Amazon.com ✓ FREE DELIVERY possible on eligible purchases
Don't know if it worth the hassle and cost by time it's all said and done. Plus you're really limited in the time frame you can use this setup.
Have a good day.