How would they build an aqua farm? They can only hunt (or fish)
How could they paint? They can only nudge.
How could they do sign language? They can only squeak and flap.
They're stuck being nomads for life unless there's a better way to talk to them?
See below:
https://www.youtube.com/watch?v=25LYVxTUZhM
Farming arose in response to scarcity. Dolphins live in abundance and only take what they need. Humans horde.
How could they paint? They can only nudge.
In the sand.
How could they do sign language? They can only squeak and flap.
Why do they need to? Are any dolphins deaf? Regardless we don't know they can't. Perhaps they can see many details in their physical movements than we think are relevant.
They can paint in the sand that easily disappears? I'm talking about permanent pieces of art tho.
The point of understanding dolphins is so that they communicate in true language. Now it is possible that they are, see youtube link above about understanding rat squeaks.
Also, if there's too many dolphins, there's starvation. Storing food would see them through.