i've had great success with a green colored sea grass common to the hobby under 5 watts per gallon. but, i would say this amount of light isnt enough for most sea grasses since that which i'm fimiliar with grows in very shallow waters...
the output of the sun in shallow waters ( from my experience from living in Florida and going on Carribean crusies ) keeps these grasses alive and well... we aquarists have to settle for what will survive in our synthetic enviorments which, of course, limits our success.