As long as your Seafaring race is active, you
may conquer the Seas and the Lake,
considering them as 3 empty Regions. You
keep these Regions even once you go into
Decline, and continue scoring for them for
as long as you have tokens there. Only Seafaring races may
occupy the Seas and the Lake.
Notice how there is 3 water regions on the map. Don't you think it would make sense both thematically and in the rules that it just means you can consider the Seas and the Lake as empty Regions for conquering (2 units + 1 for defense)?
Word of advice. Rules are usually pretty clear. If they actually intended for you to get bonus points/need more units for each region they would simply say that than bury it behind a confusing definition.