Seaweed do not have to live in water all the time - think of inter-tidal periods when they are not in the water. Only some kinds of moss live in the water. Many kinds of mosses never live in the water - but do tend to live in a damp environment. The underlying concept to think about is dessication.
There is a diverse amount of foods available at the Galapagos islands. There is a variety of seafood, as well as root produce such as onions and yucca. There is some sea-related produce they use such as seaweed and sea cucumber.
Seaweed belongs to one of several groups of multicellular algae. Seaweed does not have a common multicellular ancestor, and is a colloquial term and lacks a formal definition. Green seaweed belongs to the genus Caulerpa, brown seaweed to the genus Fucus, and red seaweed to the genus Gracilaria.
No, seaweed is not endangered. There are many varieties of seaweek and they are all thriving. If the ocean temperatures rise or the water becomes less salty as a result of global warming there is a chance that some seaweed might die out.