Sea Magic is a line of Mikimoto pearls that are not of a quality able to carry the actual Mikimoto brand. There are 5 lines in total. They are as follows:
Mikimoto (The high-quality line)
Gordon's Classic collection (this line is only available in 'Gordon's Jewelers'.
Sea Magic (available at a lot of chain-jewelry stores in malls).
Zales Signature Collection (available in Zales Jewelry stores).
The lines that are not Mikimoto are not bad-quality pearls, they are just definitely not high-end Mikimoto, and are usually quite inexpensive. Sea Magic line is a decent pearl, above commercial, but the nacre will not be as thick and the strands are sure to have higher inclusion ratios.
I would like to add that the all those lines are actually lines of 'Blue Lagoon, Mikimoto'. They are basically lines within that line. It is more or less just a way of branding them separately as far as I can see.
From what I know of the pearls they are good pearls, but not 'very' good pearls. They are a good buy for people that want the 'name' Mikimoto, without the price tag. Still more expensive that similar 'un-branded' pearls, but without the price tag of true Mikimoto pearls.