In my opinion, an LMD (line management device) is absolutely required if you want to maximize your casting distance. Water, whether salt or fresh, adds surface tension to the line. In the salt, you also contend with moving tide and weed, too; the LMD gets your line out of the water. The minimal impact you're noticing to casting when in freshwater is probably because you're fishing with floating line; salt, you're probably using intermediate. You can figure out the different impacts to your casting. To me, if you're trying to maximize your casting efficiency and distance, an LMD is an absolute must when beach fishing.