I decided to spool a couple of my offshore rigs with high-visibility line, mainly for ease of identifying my line for untangling when trolling or on party boats.

How much of a deterrent is high-viz line for fish like kings and ling nearshore and tuna offshore? Is using a long clear-line (fluoro) leader pretty much mandatory, as opposed to attaching jigs directly to the bright line?

I wouldn't.......

Just make sure you always bring your yeti and white Oakley�s.

I use blue or yellow braid and connect flouro leader via an FG knot. For tuna, you never want to connect directly to braid when jigging...their eyesight is phenomenal and you'll get 3 times the hookups on flouro leader. I've seen numerous fish turn their noses up at a live bait on a hook tied straight to braid, but rarely on one tied to flouro.

I like to use a different color line than is supplied (usually clear) when fishing on party boats. It allows you to single out your line quickly if you get caught up in one of those big tangles. Dark green or blue will works great for this purpose. I used to use bright green or electric blue. The effect on the fish will vary from day to day. If they are actively feeding the bright line is not a problem. If you are not getting bites and others are then add flouro or whatever leader is needed to get you hooked up. Many of the anglers I fish with are going to the bright measured lines. They are great for getting your bait to the right depth quickly. I still have some electric blue big game line I stored properly and the last time I went to the coast I hooked up 17 kings with it tied directly to a 12-inch steal leader but they were actively feeding.