Within walking distance in Sosua, I would go to Waterfronts. If you have transportation, then, I would suggest Ocean Village. Both these restaurants offer ambiance as well as good food and decent service. However, you will pay a bit more. For a special birthday dinner, they both are worth it.
My last time at Waterfront was Valentine's day...all I'll say is that it was my last time at Waterfront...and I used to love the place for a couple reasons...It really depends on what the birthday person likes. Morua Mai- good but expensive...check the calendar in a week there'll be a new offering...honestly, Cabarete has better restaurants and values.
I would go to Peirgiorgio for sunset cocktails at about 5:45 pm - the margaritas are good there. Then to either Casa 21 or Al Porto in Sosua Ocean Village. Get one of the Cabanas on the seafront at El Porto and they will bring everything down to you.
Casa 21 you will have a quiet romantic evening but they are not open Monday and Tuesdays for evening meals. You will need to get a taxi for both.
Waterfront is not so good any more and even their margaritas taste as if they are made with mix rather than fresh ingredients.
JR is right - El Toro has closed. Scotch and Sirloin is not romantic at all.
There is possibility Josephs at Sosua By the Sea next to Casa Marina.
I think it depends what you want - sea view at Sunset and a great meal at somewhere else. Depends how much you want to pay too. None of these place are cheap.
Sabatini's restaurant was the a la carte restaurant within the Sosua Bay/Victorian House hotel. Unfortunately it has been closed for about six months. It was pretty good with mainly Italian dishes offered. No idea if they plan to open it again. I definitely miss it.