>>213392715
There aren't hard-and-fast rules, but most piers are significantly elevated and primarily for foot traffic (not boat traffic, if you're familiar with the term "boardwalk" this is what most piers are), while most docks are closer to the water line and for access to boats. Pic related, this is Santa Monica Pier which is a popular tourist spot in Los Angeles. It's elevated maybe 30 feet above the water line, regardless of when the tide is in or out. There are similar tourist attractions up and down both coasts such as Atlantic City in New Jersey.