Definition: The reason the rules differentiate a "lateral water hazard" from a "water hazard" is that in the case of a lateral water hazard, the option to drop behind the hazard does not exist. Because the lateral water hazard runs along the side of the playing area, not across it.
The official definition of "lateral water hazard" is a water hazard situated so that it is impossible or impractical (in view of the committee) to drop behind it - as you would do with a normal water hazard.
The options for playing after hitting into a lateral water hazard are described in Rule 26.
