If you like the mariner, there are two casters who really get along with them.
Broadsides Bart is one of the Privateer 'casters and is all about making cannons even awesomer. He does well with multiple Mariners.
Magnus the Traitor isn't quite as awesome with them, but has spells that increase their durability as well as their guns' range. Also, he has a cybernetic arm. AWESOME.
Outside of these two, all of the others are mediocre with them. Ashlynn's feat is nice with them, but better with a Mule, Drake McBain would rather have melee 'jacks but could make do with them, and the other Privateer 'casters want something more durable.
If you go heavy guns, pick up Master Gunner Dougal McNaile. He makes AOEs and shooting in general a little better. As far as units to avoid for mercs, I'd steer clear of Croe's Cutthroats, the Devil Dogs, Highshield Gun Corp and Horgenhold Artillery. The ones that are pretty much amazing and I would suggest at least looking at are Nyss Hunters, Kayazy Assassins (if you go 4 Star Syndicate), Horgenhold Forge Guard (if you go Highborn), and Aiyana and Holt.
Eiryss, Gorman Di Wulfe, and Rhupert are all worth picking up no matter what you do.
Hope this helps!
