Well... I'm sure you hate to hear it, but the sea ponies are ponies... an official pony release... They weren't like the My Little Puppy, etc. -- they were listed as "My Little Pony" toys. But the real question: do you consider the pony friends (the lion, camel, etc.) to be integral to a complete collection? If so, then the sea ponies HAVE to be counted.
But ultimately, it's up to you. If you, in your heart, don't believe you can truly say that you have a complete collection without the sea ponies, then you need to get them. If you're fine without them, don't! But I'll bet you'll keep looking for them after you complete the rest of the collection...