Ultima V: Warriors of Destiny/Britannian seas

Shallow seas
After you explored all of Britannia mainland, go back to Britain. In East Britanny shipwright you can buy a skiff, and sail along the coasts of Britannia.

Deep seas
When one of your character reaches a sufficient level of experience, pirate ships will start appearing in the seas. Defeat the pirates and conquer their ship. Now you can sail far from the coast and explore new islands.

A different map could be now useful, centered on the sea instead of the land.