I did this one on the first beach you land on at the start of the level. Wade into the water and you'll start to hear a bubbling sound. I counted that after 18 of these i'd drown so I started to move back up the beach, jumping on the 17th one.
You might not get it first time but it's what worked for me.
I did originally start on the 15th bubble sound and then the 16th unsure how long it would take to get out of the water.