The salinity of seawater does not correlate to tides. Just because seawater has a higher salinity than lake water, it doesn’t mean salinity causes tide.
Some flat-Earthers claim tides are caused by salinity. They are wrong. There are bodies of water that do not have tides as large as seawater, but their salinity is much higher than seawater.
They begin with their rejection of gravity, but they need something else to explain the phenomenon of tides. So, they invented an ad-hoc hypothesis that tides are the result of salinity. This corresponds to the very simple observation that seawater has tides, but other bodies of water do not.
But if we do more careful observation, it does not work like that. Not every lake has a low salinity. Some lakes even have a much higher salinity than seawater. Take the Garabogazköl lagoon, for example; its salinity is 10× the seawater average, but it has no tides.
The same thing also applies to the world ocean. The global ocean does not have the same amount of tides everywhere. Some have above average tide; others have lower than the average. Having a higher salinity doesn’t mean having higher tides, and vice versa. The Mediterranean sea, for example, has a higher salinity than the world average but has far less tide.
And thus, their conjecture that tides are the result of the salinity of the water is disproven very easily.