It depends on why you want the dog. I would say that, in general, get the dog you want, then adjust your sailing arrangements accordingly, not the other way around. But again, it depends on the "why".
I have a dog onboard because he is my service dog and goes with me everywhere. It would less safe for me to sail at all, without a comparably trained dog - and there aren't many of them out there. For me, the dog is the best dog for me, and everything else adjusts accordingly.
I understand that, for most people, the choice is a bit more flexible. But, I think the rationale still applies.
One thing to consider is that, almost no matter what you do, there is no guarantee the dog will ever take to sailing. My dog is a professional. He travels on planes, trains, boats, escalators, gondolas, whatever - you name it, he's done it or would do it, if that's what I was doing. But, even despite all of this training and his general fearlessness and enthusiasm, he really doesn't care for sailing. He tolerates it because it is his job. But, I would say that if I did not need him, I wouldn't drag him out on the boat, for the most part. (He does like the destination - just doesn't like the actual sailing.)
So, again, you have to choose the dog that you would want whether sailing or not. Without knowing what you are looking for in a dog, it is impossible to give good advice on what to look for in a sailing dog - or maybe more accurately, a dog who might sometimes sail.
