I would definitely look at Hells vs the Main or Lower Salmon. Cold weather boating is better, with less ice, on N/S rivers. There are too many spots along those two rivers with large southern hills/cliffs to block the sun to the river and allow ice dams to possibly develop. But then again, March is pretty late for ice dams. The N/S vs E/W aspects also affects how cold you will get boating, as you will get more direct sun on the N/S sections, such as Hells. (Lower has more N/S than the Main FWIW). But if the Owyhee is running, go there, mid week if possible.
Living in Montana, boating in Idaho