1.12 has certain custom world maker options, not sure if 1.13 or 1.14 has the same options but if you set sea level too a very high number like 120-130 (play around with it) all tall mountain peaks are now the only land on the map.
Oh yeah plus if u want animals to spawn pick a biome like plains, forest, something that spawns animals cause otherwise no animals and YES you can get Ocean Structures without Deep Ocean as long as the water is deep enough (it will be lol)
Water World you got it, but keep trying cause finding a world with mountains well i mean islands close is hard plus remember u need trees too build things, unless you use cheats then just find island u want and spawn one sapling and go from there
I was really picky i wanted islands close to 0, ?, 0 cords lol (so i can start at the center of the world) but the PC world are like 500 millions blocks Squared so center is a HUGE AREA really
Because i'm not sure on higher than 1.12 is because i play with a controller and the Joy Pad mod isn't up to date with 1.13 or 1.14 yet bummer
I hope this helping everyone looking for Island Survival 100% water worlds Good Luck and Happy Hunting