The basic setup would be to make a big old map with lots of rugged coasts, peninsulas, straights, fjords, etc. along the borders, then several archipelago throughout the middle. I imagine using 4 or 6 sheets of hex paper for the sea maps, including a few Bosporus/Dardanelles/Pillars of Hercules type narrow straights dividing some sea areas from others.
For archipelagos, I plan to just transpose the layouts of some real world ones--the Caribbean islands, the Aegean, parts of Indonesia or the Philippines.
The mainland coastal areas will have Greco-Roman, Norse/Celtic, Middle-Eastern/East African, Meso-American, and maybe Chinese/SE Asian and Sub-Saharan African type regions. Various islands may have similar cultures, or may be totally bizarre or unique.
I plan to mine any sort of nautical source material--Jason and the Argonauts, the Odyssey, the Vinland Sagas, Sindbad the Sailor, any sort of pirate stories, Captain Nemo, etc. Whether it be literature or film, I'll likely steal it. So Nemo, Jack Sparrow, Jason, and Sindbad will likely all be running around and encounterable.
I'm thinking now, start all characters at around 8000 XP, so Fighters are at Hero level, and making each player role up 4 or 5 characters at the beginning, who are all on a 'hero ship' similar to the Argo, and give them a macguffin quest that will lead them to explore the seas until they find whatever they're after, sail home, and take out the king who sent them on the quest in the first place to get rid of them. This way, everyone's got a few extra characters on the ship if someone dies. And they can pick up more replacements if they land in ports.