To those who say that the act of entering the water is their "OK" signal, I hope to see you change your mind, as this assumption can get you in serious trouble.
Without going into the obvious fact that the dive boat can make no assumptions as to the readiness or competency of divers entering the water, there are an infinite number of issues that can occur between your being okay as you step off the boat and the moment when you enter the water.
I skydive as well as scuba dive and there are a tremendous number of parallels between the two sports, with the exception that scuba diving is much more dangerous. Regardless, training and rote are essential (rather than trusting on dumb luck)
By turning back to the boat and signalling the okay signal (for me it's a one-handed knuckle to the head), you're creating a pause that allows you to break the tunnel focus of "getting in and getting down". Even if the DM or Capt on the boat doesn't see you, you've given yourself a mental pause and you'd be amazed how powerful that pause can be.
What are you taught to do when confronted by an emergency? Stop (Pause - it only takes a moment) and evaluate.
Who cares if it's not what other people on the boat are doing or if you feel it's not cool - really, who cares? Hell, I feed sharks for a living, jump out of planes and technical dive, but I wear a seat-belt when I drive two blocks, and you will always see me look back to the boat and knuckle the top of my head after I enter the water.
(Okay - so there's one exception - when the dive requires that you enter negative - but then the pause happens just under the surface with my buddy. It's just like the S-drills you do at the beginning of a tech dive)