It depends on how local we're talking about. If you build a pipe out of the littoral zone into the ocean with multiple outputs you likely wouldn't kill much of anything but a few plankton. The intake pipe is often worse than the output pipe for wildlife.

For a place like, say, the the Persian Gulf, that uses oil for heat desal and gets their intake and output from a sea so it's all littoral and doesn't as quickly exchange it's water with the ocean, of course it's an environmental nightmare. It's naturally saltier without desal because of the higher evaporation rate and small comparative inlet size of the straight of hormuz, but at this point its 25% saltier than the rest of the ocean thanks to that desal.