Once in a great while I look for new rules to spice up my ongoing Hold of Sea Princes campaign. I already use a heavily modified "frankengame" sailing and combat system borrowed from Basic D&D all the way through the editions to 5E. As I run mostly Pathfinder and now 5E, here is a couple online resources for others who want to run a 5E high seas/underwater campaign for Greyhawk or any fantasy world really.
Limitless Adventures. These affordable, handy Limitless Encounter PDFs come in sets of 10 different encounters with varying levels of difficulty and are easy to reference for combat, roleplay or skill challenges. For a sea based campaign, the aquatic or urban sets can save a DM quite a bit of prep time between planned encounters. They are always adding new sets so check in often. I'm putting out a wishlist for a jungle and island set!
For starters check out their 5E pirate weapon article, and the excellent four part series on 5E naval combat. Their publishing arm even has a Pirate Adventures 5E PDF for sale. Get on this site now if you want to run a campaign like the Sea Princes.