There are two big barriers to walking around on the sea bed like we do on land. Ignoring the obvious one about breathing, which we assume is solved, the others are:
Bouyancy means that any humans who tried walking on the sea bottom would end up bouncing along like a moonwalk1, or just floating up. They also wouldn't be able to get any traction to propel themselves forward, which means they would be very slow. You see this any time someone actually in the water tries to walk.
A human body in water also has much more drag than one in air, which means that even if you had the same forces propelling you forward, and you didn't tend to float, a humanoid would move much slower than on land. Combined with the above they would be extremely slow.That would make a humanoid ancestor easy prey for anything that was more streamlined and had 3D movement in the water. A shark would make easy pickings of one.
Fortunately the answer to both of these is the same - much more weight (or technically density). A bottom-walking humanoid would have to be much, much heavier than a human, They would also have to be proportionately stronger to overcome the drag.
Given that they are going to have to be slow, and wanting to avoid being prey for the local sharkoids, this leads the humanoid ancestors down the road of armour. If you have to be slow and heavy, you may as well be well protected. This is the direction all bottom-walking life has gone on earth.
So coming back to the present day intelligent speices, since the humanoids are already heavy and well-armoured, and given that projectile weapons work very badly underwater (drag again) your humanoids are perfectly set up for some awesome sword versus shark battles - the underwater equivalent of knight versus dragon. At least you could if only your humanoids could find a way to kindle fire, and thus do metallurgy, underwater. But that's a subject for another question.
1. Armstrong, not Jackson.