The Mariner class series is the default class series available to the Mercury Adept Piers. Piers becomes this class series when he either has only Mercury Djinn set or no Djinn at all. This class series is unique to Piers; Mia, the other Mercury Adept, is a healer rather than a warrior and has her own default class series, the Water Seer class series.
The class series, when at its highest stage of Admiral and compared to the other series at their highest respective stages, has high HP and average Attack, but lower Defense and low PP and Agility. This series' luck rating is high. Noteworthy Psynergy from an endgame perspective includes Diamond Berg, Pure Ply, Cure Poison, Break, and Avoid, and the heaviest area-of-effect Psynergy it has is Megacool. Average stats from an endgame perspective are 140% (144% if you don't factor in Luck).
|Class must be Privateer or higher.|
|→Diamond Berg at Captain class||17||ATK+92|
Psynergy wise, the Mariner is a fair class: the Cool series is powerful, and Diamond Dust/Berg are quite powerful (and visually nifty), but they lose their shine when faced against monsters that aren't weak against Mercury based attacks, and the Ply series is always a nice bonus.
Statistically wise, the class is painfully lacking. The average attack is augmented by Piers' ability to wield longswords, but the PP pool is dismal, and Megacool and Diamond Berg will quickly strain it, and the agility is nothing to brag about. The good HP pool can easily be undermined by the sub-par defense stat however (although Piers' ability to wear armor will mitigate this somewhat).
On a side note, the class does have the Break Psynergy which is a plus.