I've always been very curious about this, as I've never experienced it.
What does it feel like?
Is it like a giant sudden force that just rolls you and sweeps you away?
Or more subtle where you still are swimming but it's just not working?
Since you're in a large body of water that moves,
you don't feel anything. You just see the nearby reefs shooting away.
That's below the surface.
The places where you can feel it, are the so called washing machines - usually places where two currents collide into each other, e.g. where a channel (Kandu) ends in the open sea. I have done that many times with students, so they can experience it. Just grab each other, don't let go, watch your depth and don't fight the currents (nature wins!).
Drifting away at the surface is really hard to determine. I don't see the difference between 750m and 1000m from the boat, and the water around you seems totally calm.
During every single dive, the boatcrew kept an eye on the bubbles and/or the SMBs. I never had to wait more than 5 minutes to be picked up, except for a couple of unplanned drifts where the crew still had an eye on us.
No idea what happened on that boat in Malaysia, but they didn't seem to pay any attention at all during those 2.5 hours.