With 1.13 here I wanted to load up my old world hoping the new oceans might be cool, but I'm suck with the old ones. No algae anywhere, no shipwrecks, just fish and dolphins. Is there any way I can re-render my oceans?
Only newly generated chunks will use the 1.13 world features. Chunks already generated in your previous world will remain the same. You'll have to go somewhere in your world you have never been before to get the new features.
Option #2 is to load your old world into a map editor and delete some old chunks and mark them for re-population. When you load the edited world into 1.13 you should have new features in the repopulated chunks. You will, however, lose anything you previously had in those chunks, and there's no guarantee they will even be the same biome.
I did this for my world that was created in 1.12.2. I used the following tutorial to do it with a plugin that was written specifically for this kind of thing. I'd say the result was very good. I do have some areas where trees spanned a chunk border and are now cut off but other than that, the changes are really nice.
I went exploring yesterday for a couple hours and came back with 6 Heart of the Sea and more than a stack of emeralds from sunken ships and buried treasure. See below for the best buried treasure find.
It's not going to be perfect, but it's going to be a whole lot better than if it were from 1.6 or earlier.
World generation changed dramatically in version 1.7.2, but hasn't changed much since then, at least in terms of terrain (it's mostly been structure and feature changes). World generation in 1.13 is roughly similar, in that oceans, islands and continents are recognizable, but terrain generation has changed a lot, so features will be different (lakes, caves, ravines, etc.). And, of course, oceans are radically different now.
There is an issue with the biome temperature changing. It seems the code that's supposed to prevent cold and warm biomes from spawning right next to each other broke in one of the late snapshots, and it's still broken.
My island world from 1.8 has warm biomes in 3 of the 4 directions (2 mesas and a desert), so you'd think that the ocean in the middle of that would also be warm. And in fact, when using the same seed in the early snapshots the island was surrounded by warm and lukewarm ocean biomes. Unfortunately when I tried it in 1.13 all of the oceans around my island are now frozen oceans with far less life, no coral reefs, and a surface filled with icebergs.
Icebergs off the coast of a Mesa biome?
It seems odd that a bug like this could sneak in so late in development, but I can't imagine this was an intentional change. I haven't gotten around to checking the bug tracker to see what's up with this, so... ¯\_(ツ)_/¯
There was a small change with snapshot 18w15a where my oceans went from a mix of warm and lukewarm to all lukewarm, then a dramatic change sometime after that to cold/frozen.
So what's the point of this post? Just an FYI of something to watch out for. If you are hoping for coral reefs and instead get frozen oceans, you might want to try importing your world into a snapshot in the 18w11a to 18w15a range to see if you can get what you want.
One of the reasons I liked this seed back in 1.8 is there were 3 ocean monuments close to spawn. In the early snapshots 2 of those are gone (but the one I used for my Guardian farm is still there), and in 1.13 all 3 no longer exist.
I have already created a save of my 1.8-1.12.2 island world with only the spawn chunks (which includes most of my island and all of my main base, and the unbreakable iron farm), and my nearby guardian farm. I used the older MCEdit v22.214.171.124 and first selected the entire area from the far corner of my spawn chunks to the far corner of the guardian farm. Then I used Select Chunks to align the selection with chunk borders, and Prune to trim away everything outside of the selected area. After that I pruned the excess chunks in between spawn and the guardian farm, so all I have left in the world is my spawn chunks and the guardian farm.
Now I just need to decide which snapshot version to import it to. I get warm oceans and coral reefs in 18w11a, but I'll have to look into what was added after that to see if I'll be missing out on something I want.