I saw in the trailer some rivers so I'm going to hopefully rightfully assume there will be boats.
And maybe this is already planned but my suggestion involves sort of pirates. On the ocean there would be npc pirate ships you can destroy and loot or what not. And there'd be islands only accessible via boat that might have strong dungeons. Maybe even port towns. Possibly sea monsters like krakens that take multiple people to take down. I don't know if this is planned so that's why I am suggesting it here. Because I know this section is labeled as suggestions for future content for DEO so maybe this would be the Ocean Exploration update later on after release.
I'd love a system similar to player housing where players can buy, build and upgrade their boats. Boats could be used for fast travel to islands or other continents. Another use for boats would be exploration and specific quests that require you to traverse water. Treasure hunting with your boat might be nice as well although I'm not sure if it really fits into the 'every item is player made'. I'm a fan of pirates but only if they have cosmetics I can have or exclusive skills we could learn.
Rated number 1 in Dragon Eye Online.