While helping cook for a bake sale, I noticed something while whipping cream. The eggbeater that I was using had induced a current into the cream. This current continued even if I withdrew the eggbeater.
Coastal waters suffer an issue in which between the nutrients leeching from farms, and the nutrients in the nearby sediment, develop harmful algae blooms, choking all of the life out of them.
What if we got a large fleet of boats with underwater paddles. At a coordinated time, all of the boats activate their paddles, causing a massive wave of outgoing water. The coast is drained, then re-flooded with deep water as the wave comes back. This "flushes" the coastal water, removing the harmful algae and sending the nutrition to the deep ocean, where it can do more good.