I started a new world in the 1.13 snapshots using seed "phantom" which is numerically -608397555.
I spawned in a Savanna, but Desert, Plains, Forest, and Extreme Hills are all fairly close. There's Ocean south across the Desert, which I located by following the riverbed along the Extreme Hills.
Swamp and Roofed Forest are also not very far, towards the east.
One might want to head north into the Desert there first for a Temple to get some loot.
If you go to the Ocean, there will probably be some turtles along the beach, but I had better luck with finding turtles close to each other heading east-southeast (from mouth of river at ocean) to where Birch Forest is visible (depending on render distance).
There's a Plains Village beyond that Birch Forest.
Some of the details may change with the official update (like the temperature of the ocean biomes), but this seems a good world if one likes starting in warmer biomes and also wants to test new Aquatic features.
EDIT: I now learned that there is another large body of Ocean biome to the north. From the general spawn area, this is probably (edit again to correct) 1-2 full days walk north across starting in Desert (weaving between Desert Hills). By horse the trip is maybe half that. I found a Desert Village on a river/lake at a little over the halfway point, which may be a good waystation for the trip. The Ocean proper is further north (from the river/village area) past the Swamplands.
I have been able to preview the world in both Amidst and Mine Atlas now and structures are largely as in the previews (with usual margin or error, such as actual water levels in a River biome not represented on maps.)
If the preview of this seed is correct, Jungle, Mesa and Ice type biomes are some of the rarer or more distant. But, once one has a costal base and some resources (or uses Nether travel) they would not be as difficult to access.
NEW EDIT: Added Mine Atlas and Amidst preview maps as attachments.