Pre-req.:
Survival mode, finding two turtles

Steps:
1. Feed both with seagrass
2. One of them becomes 'pregnant'
3. Follow the pregnant for almost 3 hours, while its just making circles underwater

Currently:
It goes a bit somewhere, stops, makes a few circles and then continue -»repeat as much as you'd like, the turtles never swim to shore or lay eggs. Usually after stopping and circling it heads back the way it came.

Tried it in creative mode, the same happened.
Seems rather useless mob if you can't breed 'em.

Expected:
The turtles laying down eggs