Well, the thing I was thinking is even if you're getting like 75 gold per fish you can keep cranking them out at about one every couple minutes. You'd have to look at how much time was spent per chest/skull/ect to collect it vs how much it pays, even factoring in time spent sailing between voyage locations ect. Other methods pay larger sums, but can take a bit of time to collect them. You'd just need to do some test runs to get the gold per hour for fish and compare that to the other loot.
Fishing is definitely the thing to do, even if it's only between voyage destinations. What I had in mind is to sail between fishing locations in a galleon with the crew constantly catching fish, even if it's the lesser ones to pad the numbers. Shipwrecks are great since they have the chance to supply bait, fish, loot, and supplies as well as being a fishing site. You'd probably want to sail from skull fort to skull fort and hitting shipwrecks and storms when you see them, gathering bait and looting all valuables and fishing all you can from each site moving on when bait is spent.
It would be neat if we could get a Test fleet on a server. You could really slurp up all the loot and fish and share all the profits.