Why might eating very large amounts of watermelon daily not elevate blood sugar as much as its Glycemic Index (GI) suggests?

Its high water content ensures that one serving's Glycemic Load (GL) is often low.

The potential impact of a fruit on blood glucose levels is better represented by its Glycemic Load (GL) than by its Glycemic Index (GI) alone, particularly for high-water fruits like watermelon. While watermelon has a relatively high GI number, which indicates how quickly the sugar component raises blood sugar, its Glycemic Load is frequently low because the sugar is diluted within a large volume of water. GL accounts for both the quality of the carbohydrate (GI) and the quantity of carbohydrate consumed in a typical serving. Therefore, a standard portion adds less total carbohydrate load to the system than the GI alone might suggest, preventing immediate, large blood sugar spikes, although cumulative consumption of many servings will increase the total load.