Booked an excursion through Island Marketing Ltd, on the web, St. Maarten's Trade Winds Sailing with Passat: snorkeling on a 100 year old Dutch schooner, with lunch and wine included. Good pick up arrangements, no snafus, fair snorkeling, excellent lunch, good companions, lots of laughs. Entry to water by narrow, rusty ladder was only really difficult part.
Again, booked thru Island Marketing Ltd, and very satisfactory. Two snorkel stops, first to see turtles (in the rain), then a wreck and reef which was enjoyable and plenty long. Ocean water was warmer that the rain! Drinks were available.
Excursion we had booked on web (no deposit) fell thru for lack of numbers, so we hired a taxi who took us to a beach about 30 mins from port and stayed to wait for us for over an hour while we snorkeled a couple of times. Returned to ship for lunch.
There's a nice free trolley that runs around old San Juan, so first we walked around the docks a while, chatting with tall ship residents, then up to nearby fort for a look around (if you are a senior, take your national parks membership card and you get in free). From there we took trolley to second fort, toured again, then walked back to the port, browsing shops en route. Felt safe, enjoyed the views, maps from the ship were a great help.
Later, my husband left ship again and took ferry ride across the bay to San Juan, returned on same ferry and enjoyed the outing.