The Depth Strider enchantment doesn't seem to function properly. The enchantment is supposed to be applied onto boots and increases the speed of swimming and walking in water. I've done a little bit of testing, and I've noticed that the speed at which you move in water with Depth Strider III boots on is the same speed as not having any enchantments on your boots at all.
Yeah I think that protection doesn't do anything. I have protection 11 (total) on my armor but zombies still do half a heart of damage.
also one enchantment that ik works is fire resistance. I have fire resistance 5 (total) on my armor and I just swam across a lava pool yesterday with out any potent affects
I have protection IV on my armor and it works fine
Protection actually does reduce damage, but it's just that mobs are so broken and they still do reasonably high damage even if you have armor on. Also, the reason you were able to swim in lava for so long wasn't just because you had fire protection. The regular protection on your armor also helped, because it does reduce damage.