Best not to take any plantlife growing at the beach for a few reasons:
1- they might have parasites on them that you will introduce to your tank
2- plants that grow above ground are not suitable for growing underwater
3- some of the grasses growing at the beach are ILLEGAL to harvest and you can get arrested for picking them
Besides all those very good points ..many of our local species are cold water plants and don't survive long in the warmer temps of our reef systems....I've tried..LOL..some of the turtle grasses do well under the right conditions as well as the red gracillaria you see washed up on the beaches..tangs really like it.