I think 20mins is a good estimate. We usually stay at the Boardwalk which is a bit closer to the Studios than the Beach club is, and usually walk in favor of taking the boat. The boat is a bit quicker if you happen to catch it right when you need it...but if you have to wait for it walking takes less time.
I think the pp's directions are pretty good. If you're exiting the main entrance of Hollywood studios, you walk straight ahead thru security and past the boat ramp, veer left toward the parking lot. The path starts about where the Shades of Green bus stop is (very close anyway). It follows the same waterway that the boats use to get back and forth. When you get to the boardwalk, you can walk across the bridge towards the Swan and Dolphin and then around to where the Yacht and Beach clubs are, OR you can veer right and go thru the Boardwalk area and around to the Beach club (this might actually be a little bit shorter way to go).
hth!