Under The Boardwalk
Routing Issues
Unsurprisingly, Sims can't walk through foundations ...
... no matter how high.
Sims also can't walk through decking (as it's just a different type of foundation) ...
... no matter how high.
But Sims can walk underneath ceilings/floors (at least 16 clicks beween the floor and the ceiling).
So to get Sims to walk underneath a boardwalk, it needs to be flooring and not decking.
Weather Issues
Foundations are weather-proof, flooring/ceilings are not, so when it snows you get issues under the boardwalk.
Using foundations/decking only where you want the supports and infilling everything else with floor and then placing shrubs/bushes around the supports can hide this.
Alternatively, only build boardwalks in hoods without a winter season!
Hope these notes are of use to someone.