Rock samphire is much trickier and harder to get to, requiring a huge amount of risk taking as it is usually in high, out of the way places. Rock Samphire was even mentioned in Shakespeare's King Lear - "Half-way down hangs one that gathers samphire; dreadful trade!" Needless to say, most samphire is of the marsh variety.
The name is a corruption of "Saint Pierre" - St. Peter - the patron saint of fishermen.
Though it can be eaten raw it is delicious lightly steamed and served with butter and makes a great accompaniment to fish and seafood.