Succulents are the friendlier version of cacti; they have swollen stems or leaves, but didn’t turn some of their leaves into spikes. Unlike cacti, they are not all directly related (monophyletic), it is more a common ecological strategy to store water for future use than a genetic relationship.
You can see why this one might want to store water; the soil can’t do that for it, because there is almost no soil in the crack on this vertical rock:
But why this one? It is awash with water, living in mud for most of the time.
Well, on the one hand, the mud does occasionally dry up, but other plants nearby don’t seem to need the extreme strategy. I think it’s because of the tides. The mud is mostly salt water so it takes a lot of work for a seed plant to get fresh water from it. But it can get fresh water occasionally when the tide is out, from rainstorms. So the water it gets everyday is not useful water from the plant’s point of view – may as well be desert – and not all the rainwater can be used because the first part of the fall needs to wash away the salt water first, before the plant gets fresh water.
Well, that’s my “just so” story for the day, but it seems like reasonable speculation.
In case you didn’t get made to study it in school, the “water, water everywhere, nor any drop to drink” allusion in the title is from Coleridge “Rime of the Ancient Mariner”, still a great poem.