> Besides lacking thumbs, dolphins would also have a pretty hard time harnessing fire as a tool.

I keep reading about this, and I don't see it as a major problem.

Sure, it might take a while for them to even grasp the concept of fire. Much like it took us a while to grasp relativity.

However, once they know it to be possible, it's an engineering problem. They can build their forges above water. Holding breath to operate them (until they can do it remotely) is inconvenient but they are pretty good at it.

There's also a fair bit of construction they could do without even resorting to that. Humans can weld underwater. They could too.

It would truncate the human tech tree. It's anybody's guess what a Dolphin-based tech tree would look like, given their environmental constraints.

True, but perhaps they could harness technology we never thought of as valuable. Perhaps they could build structures around sea vents, or use sticks and weeds to build pens in which to farm fish. Many possibilities exist. They could even be doing agriculture right now and we wouldn't even know it.