Now, Goodra is a Dragon type Pokemon. There's nothing wrong about that. However, I think I know why it is a Dragon type. Let's look at its Pre-Evo, Goomy.
When you look at Goomy, it looks like a slime or a slug, right? Well, it turns out that Game Freak actually based it off the Blue Dragon Sea-Slug.
Since Goomy is a Dragon type, there's no point replacing it's evolution with a different type, so I guess Game Freak made Goodra pure Dragon because of that.
Plus, the Pokedex Species for Goodra is the Dragon Pokemon.
^That is where I found the Blue Dragon Sea Slug thing
^Where I found its Species