You should continue on with your world - it's so nice what you have started! You don't have to fill in the entire map - I've seen lots of medium and small worlds created on large maps (in case you want to add more at a later date), and they work out just fine.
One other thing to mention, not sure if you know this (I just learned this myself) is if you want dive lots, the ocean needs to be a certain depth. To do this you click on the "water level" button (forget where it's located) then click the side of the land. It will raise the water level and at the same time reduce land mass.