These bullets, when fired by a LDUUV, would be much cheaper and multipe-use than if the LDUUV fired mini-torpedoes or itself exploded as a torpedo.
(Artwork 2018 Nammo BulletIN page 8)
https://www.nammo.com/globalassets/pdfs/bulletin/nammo-bulletin-2018_screen.pdf [PDF 3.2MB]
"While traditional ammunition is either stopped or deflected when it hits water, Nammo’s 30 mm Swimmer (APFSDS-T MK 258 Mod 1) swims straight through water, thanks to a groundbreaking design on the supercavitating projectile developed in cooperation with the US Navy [with USN developing similar 30mm bullets since 1994]. Jan Hasslid discusses the implications of this new technology" [page 8]