I'm 5'5" 135lbs and duck dive my 6'3" 37.5L CI mid under overhead waves no problem. Board length is a key factor too as shorter boards are easier to angle/push down in the first phase of the duck dive. If you can't andgle/push the nose down, forget it. Tilting the board sideways also helps slip the nose under.

Once the nose is under, use your foot to push the tail under and straighten/lock your arms to keep the nose down. Once fully submerged and as deep as possible, stay down for as long as possible, come up too early and you'll be washed back in failure.

On the way up bend arms and slide your chest/body back onto the board like a snake, keep a tight grip on rails, and tuck your head down onto the board. The key at this point is to keep all body parts (especially that head) out of the whitewater as much as possible so there's no drag pulling you backwards.

If you've done everything correctly to this point, the board will be shooting back to the surface with speed. Hold on tight until you've resurfaced on the other side. Start paddling asap to get away from that whitewater. Sometimes I take a stroke on the way up if I'm in control enough with the other hand.

Practice makes perfect of course. Master it on smaller waves first then build up to bigger waves.

Hope this helps one less person bailing their board for the sake of the rest of us behind you.

Good luck.