Pick it up

 Pick up garbage and litter near beaches.

Much of the plastic and debris found in the ocean has its beginnings in beach litter. As beach crowds increase, so does the amount of trash left behind.  Even a couple pieces picked up rather than stepped over will make a difference. 

Avoid tourist trinkets that come from the sea.
Find a seashell on the beach? No problem. But the shops that sell the Coral necklaces & bracelets, or art from pretty seashells, usually get their products from locals that don't know or respect the  species. They collect live, anything they can sell to make a living. Better to stick to the hammock or T shirt to support the locals.

Find more information here.