St. Augustine is the oldest city in the United States and is well worth a visit. This beautiful little town is full of unique shops, excellent restaurants, and fantastic museums. Part of why the Spanish chose the site was because of its easy access to water, and taking your boat down to see the city can be a rich and rewarding experience.
There are several boat ramps located along the Intercoastal waterway that access historic St. Augustine. http://www.co.st-johns.fl.us/BCC/Parks_1_Recreation/BoatRamp.aspx can help you locate the best one for you. It is only a forty-five minute drive from the heart of Jacksonville to reach just about any of these boat ramps. For those with a cruising sailboat or a small yacht, you can can even make the journey by sea. Just follow the coastline south from Jacksonville until you reach a break in the barrier islands called the St. Augustine Inlet, nestled between Vilano Beach and Anastasia State Park.
Once alongside of the city, there is plenty to feast your eyes upon. The historic Bridge of Lions, which separates the mainland from the beach, can be inspected and viewed up close. Fort Matanzas, the original Spanish garrison that guarded the city from the French, is also an awesome sight from the water. The waterway is usually busy, so there are plenty of other pleasure-seekers around with gorgeous boats to gaze at as well. Keep your eyes peeled for some of the large two-masted sailing vessels that cruise these waters.
For those looking to go ashore, there are a handful of marinas located in and around the historic district where you can dock. Nearly everything you would want to see is within walking distance from here. St. George Street is only one block from the water, and of course the fort is easily accessible too. There are always the trams to take if something is too far away for a walk. I would highly recommend stopping for a meal at the Columbia, which offers the most delicious Spanish food you will ever taste.