Transport Sliding
When players first started playing High Seas Combat, they would find that their transports would sometimes get stuck within the rivers when picking up a group of units. It appeared to be an irreversible game bug. The transport was stuck for the rest of the game. The only use it now had, was to move units across the river between the 2 islands. As the game evolved, players discovered that you could use the unload button to 'slide' across obstacles. Even a row of ships. You can use the unload command to make your transport seemingly sail right over top of the ships! This same trick can be used on a variety of maps to hide safely from enemy ships, or to pass right over top of other units that are blocking your path.

Using Rivers or Narrow Waterways
In order to use a transport slide of any type, you need to have at least 1 unit on it. What you do, is click on unload (or use the hotkey) and tell the transport to unload further down the river. If you picked a good spot, the transport will slide down the river right to the point to where you told it to unload. Usually this isn't the case and the transport will unload somewhere before that point. You can re-load the peon up, get your transport off of the shore, and try it again. You only need to get down the river far enough to be out range of enemy ship fire. If you are having problems getting the transport to go anywhere, try using shorter distances for your unload destinations. You can get the transport to go where you want just as easily by using several short clicks.
Once you are inside the river, you are safe from any ships. You can let the transport sit there until you can clear out any ships that were following it, or until you need to use your transport again. To get out, you must use another piece of land that is across the map. Using either of the 2 islands your transport is in between, will not work. Click unload, and then right click somewhere across the map on a piece of land. It is a lot easier getting out, and you probably will do it with your first attempt. Unexplored areas do not affect this. Just right click where you know land is located, and the transport will still respond. Make sure you give your transport a second to get off of the shore. If you tell the transport to go to the middle of the river, and clickly tell it to unload, the unload will take precedence over the move command, and unload the peon. Just make sure your transport IS off of the shore to save those valuable seconds.

Going Over Other Units
You can use this same trick to get a transport out when it is surrounded, or being prevented access to a part of the map by a line of ships.

I've used my own units here to demonstrate an extreme case. The transport is surrounded by alot of my juggs. By using the move command alone, I would not be able to get this transport out. Because I have units on this transport, I am able to use the unload command on a distant island to slide right across these ships. The second image is very peculiar. You can see that the transport is in between the juggs which are packed tight.

You might run into water blockades on maps such as, The Spiral or The Four Corners. Smart players will use oil tankers that are doing nothing, to setup a wall across a narrow part of the water. Using this type of wall, you can catch transports that do not use the unload trick. If the transport that runs into this is empty, it has no way to get by the ship blockade.

Safe Shipyards and Transports
You can also use this trick to build a shipyard, foundry, and transport that are all safe from water attacks. You just need a river, or narrow area, the no ships can fit through except for the transports. Build your shipyard and foundry there, and use the unload trick to get them passed the narrow parts of the waterways. The picture on the left is the bottom left corner of High Seas Combat. You can fit your shipyard, foundry, and a transport in this corner. Load the transport up, and you will be all set to slide out to the high seas. When you attempt building placements like this, make sure your shipyard has enough room to pop a transport out. If there is not enough room, it will say "building exit blocked", and you will have a useless shipyard. A shipyard does not hop a unit out further if everything around it is blocked. If there is no space available immediately around the shipyard, than don't do it. If you are unsure about the building room in a certain area, you can easily test it by playing a single player game.

In-game example
Here is something that I did in a real game that involved hiding in a spot inaccessible by other boats, and sliding over top of another unit.

This is the bottom right isle of Fierce Ocean Combat. This destroyer had me locked down but I slid across that little piece of land so the destoyer could no longer reach me (a jugg can't reach you there either). When I needed this transport to load up my next group of units, I used the same trick to slide back, over the little piece of land AND his destroyer.

I suggest you learn how to do this on High Seas Combat, and Fierce Ocean Combat. There is an infinite number of uses, and makes it very annoying for the enemy to try to take you off of the water. [an error occurred while processing this directive]