Opposable thumbs are not a guarantee. Nor are tree-dwelling lifeforms, nor trees, nor thumbs, nor digits, nor four limbs. For all we know, intelligent life elsewhere might better resemble intelligent octopi using alkaline metals as their first rudimentary energy source as we did with fire.
Hell, it took 2 billion years to get to single celled eukaryotes on earth. But the earth was teaming with prokaryotes the whole time, two entire separate branches of them, too, and they seem to have sprung into existence almost as soon as the earth cooled enough.
There's still a lot more of them in terms of total weight than animals and protists. A lot.
I do think that developing more sophisticated tools under water is difficult. Once you have plants on land, taller “trees” are very probable, because they are competing for light. Once you have trees, it’s likely that animals will climb then for safety or food. Once they climb, they will likely develop better gripping, etc…
I guess another way to look at this is that life on Earth is not special (although it is still an insanely amazing occurrence).