In my recent dives, I came across with divers that are unfamiliar with the common hand signals.
Writing on a slate is way better to communicate but marine life just wont stay still at the same spot so
using handsignals are faster to talk underwater.
When the slate isn't with me, i had to shout to the diver so that he/she could understand what
was i referring too.
 
Go to this link for common basic hand signals
http://www.divinginaustralia.com.au/scuba/hand-signals.asp
 
This link shows in video
http://www.scubadiver.cc/handsignals/uwbasic.htm
 
and this link is a fun interactive test..see how well you know your sign language
Shucks i failed at the 1st question
http://www.scubasigns.com/skills/index

thanks... good site.