You are pretty much responsible for clearing in through US Customs any time you return to US after having been to a foreign nation.
There is a method of registering your vessel, getting a sticker, and doing a phone clearance when you reach your marina. I doubt that many of the boats you see are actually lining up to clear customs. If you are interested in it....either call US Customs, or if you see a small sticker on someones mast or portlite....ask them about it. Its what I''m going to be doing here soon. I''ve just read about it, so I can''t give you more detail You may also be able to find info on the Customs website.