Marine ecology is actually highly sensitive to salinity in the water - and this process increases relative salinity in the water. Whilst this won't affect the volume of fresh water available for human use, it will have significant impact on marine life living in the unique ecosystem that exists at a rivers mouth.
Rivers typically also wash a huge amount of nutrients into the ocean (from runoff, natural effluent and the like) - I have no idea how / if these plants will affect these nutrients, but as anyone who has ever fished at a river mouth will know, salt water fish follow that leading edge of 'brown' water to feed off the food washed down by it.
There will be an impact and - similar to Australian plans to pipe fresh water from parts of Australia with plenty to those parts with very little - there seems to have been very little analysis into the impact on marine ecology, and until that has been done, one can't say there won't be any impact. Why do people forget the fishies?!