Question first: Should activity level, size/LBM, and water consumption affect salt intake?
I'm 6'2", 200lb, and row 1-1.5 hours per day, 5 days a week in 70º or greater. Needless to say, I perspire fairly heavily during workouts. I drink at least 3-4 liters of water per day, and my sodium intake is low. Will adding more salt to my diet improve hydration or water retention/use?
Un-medical opinion: yes.
At a strenuous 1.5hrs of exertion, you have likely also depleted glycogen stores, as well as copious sweating. Drinking water is fine - drinking water while exerting yourself over great than 1 to 1.5hrs is right on the edge of iffy - you need to get some salts too.
Cordain's "Paleo for Athletes" has suggestions for hombrew "sports drinks" - basically sugar, water, salt, flavor. I'm not advocating grabbing a Gatorade, but since you're right on the edge of glycogen depletion after 90 minutes, a hombrew sports drink (with salts) sure sounds pretty appropriate.
Peeing more at night on Paleo? 3 Answers
New take on water fluoridation 5 Answers
Hack My Rehydration Salts! 2 Answers