Let's do some calculations...
Dandin's boats are great for scooting up and down the Shining River, but they'd have to hug the coast to be safe at sea. It would be a bit dangerous (check involved), and It's about sixty miles downriver to the sea, which you could easily do in about 10 hours, but I don't think you'd want to sail at night. (You could end up out to sea, or wrecked on shore, both bad things)
It's about 120 miles up the coast to Waterdeep, but you'd have to follow the coast closely, which goes in-and-out, so probably more like 200 miles. (In particular because there's a few big rivers that you could follow up before realizing it, or be pushed out to sea, so it's a feat of navigation, too, as well as seamanship) So, you'd camp about four nights, assuming all went well.
It could be done in five or six days, barring mishap. Then back. Not really faster than riding, and more dangerous, but Dandin could carry a lot of goods. Take Janus with you as your expert. He may be a riverman, but he knows his boats. (He was the defacto riverfolk leader who's family you saved at the beginning. He likes you guys.) He'll take a reasonable cut, and make sure you have a brave crew.