Yeah, Dragonite lives in the water and helps boats, so it makes sense....*depressed sigh*I think that its based around 1 of 3 reasons
1. If based on coloring, the wings are a teal color, so it could go between green and blue(also dratini and dragonair are blue).
2. Dragonite's name in Japanese is Kairyu, and the kanji for "kai" CAN be translated as ocean/sea, connecting it to water more.
3. Could be due to the fact that dratinis have only ever been obtainable by fishing.
About the Prism star card, you did not dream that you read that!....But it was just a joke made by someone before the actual text was revealed. Sorry, better luck next time!