>why would they boast about it and invite attacks upon that weapon and its deployment systems?

To complicate adversary targeting priorities. If you have to shift your pre-planned bombing sorties away from, say, local Basij HQ buildings, it takes pressure off of the Iranian government. Assigning aircraft to find/fix/target/track/engage "underwater drone launch points" is probably like searching for a needle in a haystack given the size of Iran's coastline.