Fresh water is less dense than salt water. It will therefore float naturally on the sea if separated by a flexible, impermeable membrane.
INDICATIONS
If a 'floating freshwater lake' structure was built with a flexible surround ring and bottom membrane, like an enormous rubber dinghy, fresh water retained in the lake would rise above the level of the surrounding sea and keep the surround extended.
Inflating the surround ring, as with a rubber dinghy, would also serve to keep it extended.
Multiple concentric surround rings would reduce salt spray contamination in the inner lake.
Flexible construction of the floating lake should give it good stability against wave motion etc.
A large lake could be broken up into smaller, linked or independent, cells.
Cells could be covered with inflated-rib roofs, perhaps with sensors to divert salt-contaminated spray into the sea beneath.
The membrane at the reservoir bottom should be at pressure equilibrium, so minor holes should not greatly leak in either direction.
Development of devices and control software is possible here, with new approaches open to patenting or trade-secreting.
* * 
* * 
* *