All around the lake, there are several spots with boats offering 1h30 tours on the lake to tourists. The price is per boat or per person. The main pier is the one I give GPS coordinates below but there is one on the other side of the lake, just after the town of Khumphawapi, which is less well-known and where the locationfrom where the boats leave with the fishing huts is more beautful and much easier to reach by public transport followed by a tuk-tuk(see below). Moreover, wherever you choose to start your boat ride on Udon Thani red lotus sea, I recommend you to go to Khumphawapi city too. In the middle of the city, there is a park with monkeys in freedom, it is always fun to see.
I muss confess that over the years the lake which was almost entirely covered with lotus saw the surface they cover reducing drastically every year. Tourism activity and the choice to use motorboats rather than rowing boats is probably a big factor. If nothing is done, I fear, there will soon be more flowers to see at all. Let's hope that responsible management of the ecosystem of this exceptional place will be quickly put in place. From my point of view, it is urgent!
To go visit Red Lotus Sea, you can also book one of the tours above.