Balloons are detrimental to marine mammals, sea turtles, and birds because they can oftentimes be mistaken for jellyfish or other prey (plastic bags pose the same problem). Marine animals ingest this non-biodegradable material (typically rubber or Mylar) and then can't digest it. This causes intestinal blockage, which ultimately leads to starvation. Not only can balloons and other marine debris be ingested, but it can entangle marine animals as well.
So next Valentine's Day, or any other holiday, avoid purchasing balloons or at least be sure they are disposed of appropriately and do not end up in the marine environment. In addition, if you see balloons or other marine debris out on the water or on the beach, if possible, pick it up and dispose of it properly. To read more about the problem of marine debris click here.