How do I get passengers on my roller coasters?

Crashes have a major impact on both the ride, and the overall Park, for a few (in-game) months, and even with free entry, or free rides, no one will be willing to ride the affected attraction.

Source

The best bet is to completely remove the entire ride, and replace it - this can be with the exact same ride, but again, your Park will not have many guests that are willing to ride the attractions in the coming months.

As for the issue with the boardwalk, it depends on how long it is. From memory, I believe guests will turn around within 1 "block" of the attraction's queue entrance. if your boardwalk is only that long, that might be the cause. The other thing to try, is pick up a guest, and place them on that square, and see what they do. You can also click on guests to see what they are thinking - some may have thoughts about that attraction in question.