The amount of energy we'd need to pull from those flows even to power the full need of the world is likely pretty trivial compared to the amount those flows actually contain. Oceans are world engines, literally. There's a lot of energy in there.
These wouldn't prevent flows either, just leech a little and slow it down. Worth exploring.
The biggest issue is upkeep. Oil rigs require a massive amount of upkeep because salt water really wants to destroy most structural material, fast. Then you factor in costs of getting crews out there for maintenance, etc .. it adds up. Costs not just financially but in carbon as well. Material advancements help but you're back to the carbon costs with those in a big way too.