Diesel cruisers like GBs and the like are, in my opinion, not very well suited for trolling in most of our waters up here. Even at idle rpm they tend to go too fast for downrigger trolling for kings. And running the diesels at idle for hours on end is not necessarily good for the engines, depending on what kind of engines the boat has. We do a lot of salmon, halibut, and ling cod fishing over the course of a year and none of the places we fish are places I would want to take a boat as large as our GB. We are often right up against cliffs or in amongst rocks, reefs, and islets, sometimes in pretty strong currents that carry you toward the shore or rocks.
If one fishes in more open water for halibut, like the charter boats out of Homer, AK for example, then a larger boat is fine obviously as witness the size of the charter boats. Of if you're into jigging for salmon on the big shallow bars like Possession Point here in Puget Sound. But close in to shore we've found it's really nice to have a more nimble, maneuverable boat. And one that's fast, too.
There are ways to slow a cruising boat down some without running the engines at dead idle all the time. Trolling valves on the transmission(s) is one way. Dragging something--- buckets, sea anchor, drogue, etc.)-- behind the boat is another. Coho tend to like a faster lure but for kings the best lure speed seems to be about 2-3 mph which for most diesel cruisers is pretty much dead idle.
The people we know who really like to fish while on a cruise use their dinghies, and the ones they have are large enough to do the job properly. Boston Whalers, Bullfrogs, etc. I use our 9' Livingston for ling cod fishing among the rocks and little islands and for crabbing. But only in protected waters. It's not suited for trolling in the often-choppy Rosario Strait, for example.
But in almost 30 years of fishing Puget Sound and BC, we've found that the best way to do it is with a purpose-built fishing boat. In our case it's a 17' Arima (photo) but boats up to about 25' work really well. You can fit them into tight spots, you can get right up against a steep shoreline or cliffs, and you can troll out in the open waters even it it gets choppy. You can play a fish no matter where it goes in relation to the boat without the worry of obstructions like cabins, masts, booms, stowed dinghies, keels, deep props and rudders, etc. You can go relatively economically with something like a used C-Dory or spend a fortune on a new Grady-White.
The last halibut I caught back in June was 100 pounds (photo). Even with the fish right beside us it was not an easy task to gaff it and get it in over the side of the Arima. A boat like our GB has too much freeboard to easily gaff and haul one of these things aboard unless you have good strong help. If for no other reason than the hand rails would get in the way.
And even though we usually bleed a halibut before we bring it into the boat the end result is invariably a big bloody mess as the fish flops around. Easy to sluice it out with buckets of water in the Arima. Having the blood and slime on the aft deck of the GB where it would drip down into the depths of the lazarette and so forth would not be so easy to clean up. And the GB has too much freeboard to be able to reach down and bleed the fish anyway.
You can certainly use a cruising boat for fishing but an aft-cabin boat like our GB would not be a good choice in my opinion. There is the freeboard issue and the aft deck is not all that large. A GB32 with its single engine and step-down aft cockpit would be a better choice over our boat if one was bound and determined to fish these waters with a cruising boat.
I only know it as a Pompano that was great eating. I caught it while trolling from the dinghy with a Rapala around some rocks in Tenacatita, MX. Now I know, thanks.
