Sea skimmers present special issues for detection and intercept, the assumption that a weapon system is only useful against ships of the line is a bit narrow minded with out oilers and supply ships a fleet at sea does not stay there long.
That is the problem with subsonic stuff, lots of time to shoot it down once detected, regardless of which ship is being targeted.
Our navy sure as heck practices shooting at "crossing" targets, IOW stuff aimed at another ship, like an oiler or whatever.
I dunno, without a supersonic dash, I wonder about the ability for any of these to score a hit against modern ship defense stuff. The "CIWS uber alles" crew here on FR seems to think it can deal with Mach-3 incoming with no damage to the ship, so Mach .8 should be trivial.