You may enter near the land side of the pier and snorkle. Sometimes there are literally thousands of fish there. There are even several species of seahorses. People scuba dive here and I have seen people night diving here. When cruise ships are docked, access to the pier is prohibited to ship passengers. You could still swim over from the shoreline to the pier, but there are often small tour boats going in and out, so that could be a bit dangerous. Ships are only in for a day and usually depart by 5 PM, so you could always wait until then.