He never said he can't hurt Marduk - in truth, and Marduk should be smart enough to know this without being told, it's solely a matter of who gets the drop on whom; it'll come down to that between pretty much anyone (Surprise, so no defense; called shot for +4 DV, first shot deals around 15P damage and the second around 13.33, make that 11.67 and 10 damage after soak... Marduk's not surviving that). The difference is that Jimmy has ways to kill him and avoid any consequence, and that while neither of them is the biggest gun in town Jimmy's the one with the resources to be able to fight above his weight class, so to speak. Marduk, on the other hand, would have his life collapse and then die - which he doesn't seem to realize the full extent of.
As for the whole "fish" thing, I think this is the first time he's actually used the term out loud - Marduk probably doesn't even know what he means by that.
At the end of the day, though, he should at least be able to recognize that he's probably better off following along with the ridiculously connected guy that can get him the things he wants, can get his contacts to try to find what he came to Seattle for, and who can massively increase the amount that they get paid. And that he clearly has his own rules about "settling business" - if Marduk refused to work with anyone who didn't follow his ways of dealing with such issues, he'd never work with anyone.