In beta I would have agreed that sailing was a great way to earn gold, but they really toned it WAY down since then. You really don't make that much money sailing anymore.
There are some things you can do to increase you gold intake while sailing or other times...
Before you go sailing get the 2-3 quests from Bart (something or other) he's in Port Royal in town cowering behind some boxes. He should have 2 or 3 quests for sinking ships, and oddly enough you can finish most of these without even sinking the kinds of ships described in the text. They are worth 35g each, so you make a cool 105g extra just for grabbing a couple quests on your way out to sailing.
Sam Seabones (also in Port Royal) gives 2 repeatable quests for Navy Officers and Skeletons, which are right up the road from him which are worth 25g each and can be done in about 3 mins, or even faster if you have a friend hang out at Sam and let you teleport back to them.
I've never been a gambler so I'm glad some people can make money at it, but for me I like the sure thing
I know there are a lot more quests out there, but those 2 guys are the simplest and quickest I've found so far.