For all these answers, every one omits fire. Fire is absolutely vital, yet none but a few who post in comments consider it.
Here is the crux of the matter. Fire is vital because of the energy boost involved in consuming cooked food. The human digestive system is 25% shorter and consumes proportionately less energy than the immediate primates, and is attuned away from the ability to digest cellulose at all. Cooking also kills numerous parasites. The full benefit of this would not be immediately available, but this puts rotting meat back on the table with far less immune system energy consumption. The changes involved free up energy for the intelligence required to wield fire.
Fire is at least four steps in toolmaking. That is it is the equivalent of making a tool to make a tool to make a tool to accomplish a task. This in turn requires the intelligence to understand the steps, and the grasping hands to manipulate fire, and I'm pretty sure the loss of any significant amount of fur on the arms.
(Wait what you say. Fire is vital for intelligence but intelligence is vital for fire. Yes I know. 1022
No wonder people don't want to talk about it. The claims are simply not made, but archeologists know where they find cooking fire it is human, and the ability to use fire is the best test of intelligence.)
The fine development of the grasping hand and tactile sensation and the hand-eye coordination would tend to make the hand not-so-suitable for walking on (you want arthritic hands in short order, start developing the knuckle-wakers for fine work). This means unless you started with six limbs, you end up with bipedal. None of this, however, required an internal skeleton at all; however in this world there are reasons there aren't any large exoskeleton creatures.
In answer to other claims, intelligence is the game-changer. Humans are the uncontested apex predator in all terrestrial environments that can possibly support a human (we can't live on insects), and now in the shallow oceans.
You ask for a potential space fairing race. This immediately doubles-down on the need of fire. In addition, while I like the idea of an acquatic space-fairing race the essentials of setting one up appear insurmountable. Let us suppose for an instant the water-world with only a few islands, yet they somehow grasp for the stars (for them space is an ocean would seem more true than even to our storytellers). But the first building is the VAB. How do they refine metal enough to build? How can they ever discover rocketry without first the need of gunpowder? How could such a struggling race manage somehow to lift the first of their own with all the tons of water required (although I must say this makes recovery much less to fear). Once put the details to it, it just seems too hard.
If you want a non-bipedal you are likely to end up with something like a large dog with an extra set of arms coupled to the skeleton with another set of shoulders just behind the ones for the forelimbs (before the forelimbs would likely be too front-heavy). 335