Be VERY careful when you select a spot for snorkeling off a pier, especialy piers that are located in town where boat traffic is at it's heaviest.
Look first for the boat traffic lanes and for where boats moor - stay far far away from these spots, and every time you so much as hear a boat, poke your head up and look to see where it is, and where it is headed. Regardless of any notion you may have about swimmers and right of way, the fact is that if you want to live you need to get out of the way if a boat is coming towards you. You cannot rely on the idea that a boat dirver can see you - especially if you are swimming where you should not be swimming. I have seen a lot of people snorkeling in dangerous spots - and unfortunately I witnessed one person get run over by a boat. He had been repreatedly told not to go outside the marked swimming area, "but" he continued to do so - his third time outside the bouys ka-bam. This man lived.
Another tragically unwise person dedided to snorkel at the Texaco dock a few years back - he died.
Personal advice for maximum safety - do not snorkle at a pier during the peak boat-travel times - those times around resorts tend to be 8:30-9:30 am / 11:30-12 (mid-day) 2-ish and 4-ish in the afternoon.
There is veru little boat traffic from 12-2 pm, and it's a nice time to be in the water.