We have taken the earliest ferry to Anguilla several times. If you get there about 20 minutes before departure, you should be fine. Don’t forget to bring your passports. I’m not sure if they take cash or cards but I think it’s only cash .if you were going on a day where there are a bunch of cruise ships in and there is a market by the port parking is a nightmare so you would be better off to take a cab. I do believe the cabs only take cash, however, all the restaurants take credit cards. No euros just US dollars.
Shoal Bay is beautiful however I would recommend going to Meads Bay, specifically to Blanchard‘s Beach shack. Last time I was there a chair rentals were five dollars and they would apply the fee to your food bill. The food is very good and reasonably priced and the beach is beautiful. As early as you are going you’ll probably get there before they open which is I believe 10 o’clock, But just take one of the chairs that are out on the beach and you will be fine. As you are facing the ocean to the right, there is a decent little beach bar about 200 feet down. If you are looking for something higher end continue to the right past (red roof) to the end of the beach and go up the steps to the Maiuihana .Very nice restaurant bar and pool area but much more expensive.
If you want to hit Shoal Bay would do so in the afternoon as most of the beach bars there don’t even get going until lunch early afternoon. I would highly recommend Tropical Sunset , However, Gwen’s and Elodia’s are also good. Service at tropical sunset is quite a bit better than the other two. If you find a taxicab driver at the port you can arrange for them to come and pick you up from wherever you are at specific times. I have done this several times and they always show up.

While Anguilla is somewhat more expensive than Saint Martin, The places I have mentioned are more budget oriented and the food is very good. The beaches are beautiful, the people are friendly, and there is no traffic. Have them pick you up at least one hour before the ferry departure you want to leave on. I would take the last ferry back to Saint Martin. Also, of note is the cell phone service on Anguilla is different than Saint Martin, at least the French side. However, if you have a phone issue the people at the restaurant I’ve mentioned are happy to call a cab for you