Hitchhiking a boat
There's always the option of taking the sea route between two places. People have taken advantage of the seas for as long a time as civilizations have existed. Boat hitching is propably more like working on the boat and paying for your fare that way. It is difficult to get onto harbour facilities. It is great when you know someone on the terrain. Ships and boats don't steam off every minute like cars. So be prepared and know what liner you want to get on. One could also write to shipping companies.
Hitching a ferry
Hitchhiking a ferry is easy. Simply ask drivers or truck drivers if you can board the ferry in their truck, they usually only have to pay for the vehicle. See also Category:Ferries for different routes that involve ferries. It's advised to find out in advance if they charge per vehicle (good) or for each passenger as well (bad).
Hitching a boat
Hitchhiking a boat can be quite an endeavor for some but can be done if you put the right effort in it. Your best chances are to become part of the crew. In the Caribbean it's not too hard to find sail boats that need an extra hand on deck.
With boats, the procedure of hitching is not so much about finding someone going to your destination, but more about finding the right boat and/or captain for you. In general, a lot of captains are ready to take persons (crew members) on their boat to aid on the most mundane tasks, or just purely for company.
Finding a boat can take you a lot of time. Mostly boat-hitchers try hanging out at the harbor for a week, talking to almost anyone, possibly finding a pub where sailors tend to drink their beer. Another way people often get rides on boats in the San Francisco area is to go to the pier with some beers and offer them to folks who look like they are heading out.
Be prepared to change your schedules and plans accordingly. Boat rides are not happening every day, and it can take days and days to reach your destination, unlike with cars. The weather can be unpredictable. All these things together mean that you can't really plan on arriving at place X at time Y. Time takes a different shape and form on the seas.
Perhaps the best way of encountering a suitable ride is to visit popular marinas that outbound captains frequent. They are probably there for overnighting or waiting for suitable weather and winds. Or possibly they're short of crew because of some force majeure (people change their plans and get seasick and so on...) This might mean that they could actually need your hand on board.
Be helpful and interested in their trade. Even if you do not have any experience in sailing, be honest with the fact and state that you are eager to learn. Show respect towards their skills, their boat and the seas and the elements of nature. There are lots of good captains out there, with good intentions. But there are lots of different "classes" of captains out there, lots of different boats for different purposes. So how do you find the good captains out from the hasty, less benign ones? Well, start by analyzing how well they care for their crew. Or their boat. How prudent they are about security. What kind of personal imbalance they might be experiencing based on their insistence on getting certain types of crew (like young, good-looking females).
Then there are some websites that can be helpful for getting the right crew and boat find each other. I would recommend Crewseekers.net  since FindAcrew.net  fools either you or the captain to pay exorbitant prices for the ability to communicate.
Hitching a Barge
Some inland routes are navigable. Big rivers, canals... Barges and private boat cruise them. Barges are very long and flat boats that can carry thousands of tons of goods, slowly along the river. If you're not in a hurry they are easy to hitch.
Barges won't stop if you wave at them from the bank. The best place to get to them is a "lock", where they have to stop to be adjusted to the level of the river-canal. But they usually have a restricted access.
Apparently they do not operate at night (10pm-6am). If you go to the main gate, there should be a button to call the operator (only one person is usually operating). From my experience in Germany, they are very hitch-hiker friendly. If you explain to them what you're up to over the intercom, they let you in or even ask the boats for you over the radio.
Traffic is low but the boat drivers are usually open to travellers. And during the time needed for the lock to operate, it is easy to talk to them from the bank.
The cruising speed of a riverboat is circa 13 km/h. But counting the time spend in the locks, it can go down to 6 km/h. But as they stop only at night, it is still faster than walking.
Plus, given the size of the vehicle, it is very comfortable. And depending on what river you're cruising, it can also be very beautiful.
- Cruiser Log, has some very useful info, some specific for hitchhikers, including a rough guide of movements of cruising yachts
- 7knots has a practical way of finding boats
- Boating OZ, mostly in Australia, some Pacific
- aferry click on the map to see lots of ferry connections from all over Europe
- Boat hitchhiking, a dummies guide
- A skippers' point of view on people looking for a boat
These sites might prove useful:
- CS Group: Sailing - Sea, Sun and Blue Sky fanatics
- CS Group: Vagabond Sailing
- CS Group: Couchsailing International
- Desperate Sailors
- Noonsite - For seasonal info and ports etc
- Yachting and Boating World Forums - Crewing Opportunities Forum
- Sailing Anarchy - Crewing Opportunities Forum
- Cruising Sailor - Crew Positions
- Desperate Sailors (free crew, boat, services and event finder)
- World Cruising Club
Weblog of two Dutch students hitchhiking across the world by ship: