Mckenna, J: Dangerous Waters

The Archmage rules the island of wizards, enforcing a ban on the use of magecraft in warfare. But there is a rumour of rogue wizardry abroad, its adepts not subject to the Archmage's edicts. The Emperor of Tormalin is offering them his protection while corsairs are raiding the coasts and merchants are begging for magical aid. The world of Lescar faces a terrifying change and epoch defying adventure. The Archmage rules the island of wizards. From here he enforces the Edicts of the Council of Wizardry. Foremost is the ban on magecraft in warfare. But there is a rumour of rogue wizardry in Lescar's recent civil war.
There's the rise of Artifice, its adepts not subject to the Archmage's edicts. Now the Emperor of Tormalin is offering them his protection. There are corsairs raiding the Caladhrian Coast, enslaving villagers and devastating trade. Barons and merchants beg for magical aid. But all help has been refused. This is no comfort to Lady Zurenne whose husband has been murdered by corsairs. Now a man she doesn't even know stands as guardian over her and her daughters. Corrain, former captain and now slave, knows that man is a rogue wizard, selling his skills to the corsairs. If Corrain can escape, he'll see justice done. Unless Jilseth, magewoman and Archmage's confidante, can catch the renegade first, before the full extent of his villainy is revealed. If that happens, at a time when wizardry faces so many other challenges, the scandal could have dire consequences indeed!
Dangerous Waters is the first book in a stunning new fantasy series from a major voice in fantasy.