I agree with you on all your points on DR. You certainly wouldn't consider swimming in the sewer (I mean the harbor there) but you can dingy out around the corner at the entrance. That was a daily trip for me with my dog and the water is crystal clear there.
I went there for two days. I left a month later. It's not a long term place unless you are an over-aged SW American looking for a very-young spanish girl. Unfortunatly I saw that situation too much.
If you can get past the rats and other things you will find the locals are very friendly and it is a unique place to see. I'm glad I saw it once.
I learned I was actually very safe walking the streets of Luperon. I found out the policia are heavy on any type of theft by locals on the visitors. While I was there, a yachtie left $5 on a bar (after he had been told not to) he had it stolen by a local girl. So within 30 mins there were two policia there and they went after the girl. I heard stories of other locals getting beatings after getting caught stealling from yachties. The officials don't want to loose that stream of incoming yachties. So it was safe on the streets but you still got to lock up the boat. during the month there was also a pirating incident in samana.
After you've spent a month reading Bruce Van-Sants book (and gotten a head-ache from him talking in circles) you will need to go just to drink a beer with him at Marina Blanco.