1. One may carry only up to four Amos within a very small boat.
2. A wall that does not physically reach the ground is viewed by Halachah as though it extends to the ground as long as there is not enough space beneath it and the ground to allow a goat to pass.
3. A wall (such as one over a body of water) that does not physically reach the ground is viewed by Halachah as though it extends to the ground even though fish can pass underneath it.
4. One may make an Eruv between two boats that are tied to each other.
5. Even if walls are built on Shabbos, the domain they create takes effect.
A BIT MORE
1. In order to have the status of a Reshus ha'Yachid (in which one may carry more than four Amos), a boat must have walls ten Tefachim high and be four Tefachim wide. Some boats are only this wide at the top, but become very narrow at the bottom. In such a boat one may not carry more than four Amos at a time.
2. This means that if there are three Tefachim or more between the walls and the ground, the wall is not considered to extend down to the ground.
3. This is apparent from the fact that we permit drawing water through airspace surrounded by walls built over a river or ocean (such as from the balcony of a house built next to a river). Even though the walls do not go extend down to the ocean floor, we view the walls as though they extend downwards.
4. This is so that that they can carry from one boat to the other.
5. This means that one may rely on these structures even for purposes of Halachic leniency (for example, he may carry within the walls if they enclose a Reshus ha'Yachid). However, if someone built these walls on Shabbos intentionally, then l'Chatchilah one may not rely on them (and he may not carry within them).