Unfortuantely, the ongoing fukushima disaster means that it is wise to avoid ingesting things from the ocean, particularly from asia.
The hypocrates institute in America runs nutritional education courses and make clear that the main issue for us is not so much in sourcing B12, but in absorbing and using it. That process is one which requires adequate amounts of other nutrients that many are lacking. Other doctors I have listened to have said that the idea that vegans cannot source B12 is based on old data which has been shown to be inaccurate with more modern scientific methods and equipment. None the less, we only need a tiny amount in a lifetime anyway, so a good vegan B12 supplement is simple enough to use and access. That said, there are several forms of B12 supplement and to my understanding, only one of them is actually beneficial - the ones typically found in supermarkets and other shops are not particularly effective and may even be counter productive.

As an aside, the only person I know that has been prescribed a B12 supplement is my dad and he is a meat eater who avoids fruits/vegetables.