I'm still trying to understand where this often stated idea of non-union paying higher than union comes from... It's not just you that has used words such as 'always thought it was interesting....' to make these points. Some non-union LDO jobs pay better than some union LDO jobs at some unions on some ships. It's certainly nowhere near a blanket truth.
The GOM up until very recently has gone through an insane wage spike (which has clearly ended). I do not know about towing or harbor tugs, but I do know a guy at Genesis making $850/day plus a $20K annual bounus (guaranteed) with a 1600T MOT. Also, insane, I think. Not to denigrate his skills, but really?
When Gulfmark starts handing out pay cuts ranging from 12 (night watch mates/DPOs) to 22% (mates still training and not DP certified) (facts, not speculation or sea story) the pay comparison argument (which was nothing more than a temporary one) is short lived. When ECO starts retroactively not paying out the entire previous year's 401K match, the argument is also weakened further.
When companies like HGMI pays a new 3/M grad only a month or 2 out of A&M $850/day (fact, not speculation or sea story), I consider them to be nothing less than fools. And the same goes for anybody else who would do the same, including drillers who will hire brand new mates and engineers out of academies for a $100k, when they could have gotten them for probably $75-80K.
I don't work union right now, but when I feel the need to contemplate the possibility of MMP wages getting cut anywhere near 22%, then I'll have something to worry about.