Another issue that seems to be overlooked is lighting. Unless they all have dark vision or echolocation, they will have trouble seeing if more than 50-100 meters below the surface. And if they can still see at depths of 100-150 meters then they would likely find the surface too bright to tolerate unless they only visit during the night. And that does not even take into account the blue-shifting of the light, as red is quickly absorbed in water and yellow follows not long after. Or the fact that lighting can vary notably by latitude, sediment level, etc.
Then there is water quality to consider. Most salt water sea life, if placed in a fresh water river, will not survive very long - and vice versa. There are some fish species that can live in either, of course, but most have trouble with the transition.
Finally, consider the depths involved. Most sea creatures have specific depths they inhabit. Some are incredible divers, of course, but most have specific ranges of water pressure and salinity in which they flourish. Californian sea lions, for example, typically dive to between 30 and 100 meters, even if they are capable of diving as deep as 200 to 300 meters.
So if you want an aquatic species developing neolithic and higher tech, you may want to consider where they are located. The tropics are best for higher depth corals, while volcanic archipelagos such as Hawaii are best for higher depth lava sources. Tropical waters are typically clearer than chillier waters, and locations with low sedimentation are better for water clarity while also being less fertile for local plant life (and therefore animal life). Seaweed can exist as both plains and even kelp forests, the latter of which is more commonly found in temperate environs but can be found in both tropics and arctic areas.
As for biotech, so long as the sea folk are long lived there are many possibilities. Coral might be guided as it grows to form specific shapes. Plankton and other microorganisms might be isolated in translucent semi-permeable bladders to produce light or ink or various substances. Fish and mollusks can be used as sources of venoms, inks, silks, muscuses and slimes, chitin, shell, and so forth.
