I imagine the additional costs of cleaning/replacing tubing when contaminants accumulate from less-than-potable water outweigh the benefits (from the datacenter's perspective).

Not coastline, but the Lake Anna facility in central VA uses lake water for cooling. It's a two-loop system - internal loop touches the radioactive bits, which is pumped to heat exchangers, and an external loop ingests cool lake water and dumps warmer water back into the lake.

The "warm" side of the lake has more valuable housing because the lake remains usable for a longer period of time each season. There's a series of dams that keep the smaller warm section separate from the natural cold side.

https://en.wikipedia.org/wiki/North_Anna_Nuclear_Generating_...

https://www.virginialakehouses.com/wordpress/wp-content/uplo...