The difference in viscosity (density also changes but has no direct effect) caused by the difference in salinity between brackish and salt water(roughly 17 and 33 parts per thousand for each, or half) will have negligible if even measurable effect on the penetration of water in the relatively large openings as found in a fishing reel. So while yes, given a precise enough experiment with small enough openings, fresh water will flow faster and easier than salt water, in real world applications that is actually moot, AND, even if the flow of salt to brackish water was reduced by half (which it isn't), the concentration of salt would be the same in the water in the reel.