Darwin

No, you can't use the yacht to get there until you speak with the person on the docks. The person appears a little bit later after you do some of the story in the dessert, iirc aorund the same time you can use the Claydol to get back.

I'm pretty sure you can't get a dragon pokemon so early. If you're lucky you can get an Axew from the mysterious egg around the 4th or 5th gym (can't remember correctly atm), but this is predetermined and you can't influence what will hedge from the egg. The other dragons are only obtainable very very late, Deino for example just at the end of V12. Ralts is obtainable rather late or more in the late mid-game, but much earlier than the dragon mons. If you habe trouble with rock types, I recommend to get the Buizel in Gearan City.