I didn't do Nouakchott - Dakar along the beach but I did ride from the Mauri border to a beach north of Nouakchott, camped there in the wild and then the next day rode all along the beach to Nouakchott itself. This was the most fun day in a 6000+Km trip.
The sand was hard enough to be able to ride around 80Km/h. The closer you go to the water, the firmer the sand is - just watch out for tidepools.
Make sure you know the times of the tides! I left around 6AM and you only have a few hours before the sea comes up too far and forces you to ride in very deep sand, pretty much impossible on an overland bike.
After spending a night in Nouakchott we rode to Lac Rose in one very long day. Maps and gpx tracks can be downloaded from my site here
Anyway, long story short, this pdf file should have the waypoints you are looking for.
It's from the Zebrabar
, which apparently is a good place to camp & get info. I'd also advise you to take the beach route I did, it's a great ride.