This is one I caught the tail end of, either in the late '80s or early '90s, and looks like it was made in the '70s...
The way I recall, two girls are scuba diving, & looking for a Nazi submarine... The girls are talking through underwater radios, even though this obviously wouldn't work, as they both have regulators stuffed on their mouths... Poorly dubbed as well...
The scene cuts to a woman in a black turtleneck & eyepatch with a riding crop tucked under her arm... With a phone receiver, she says, "Launch my divers", & cut to scuba divers jumping off a yacht.....
I forget how the girl escape the bad guys, but it turns out they are in the middle of a navy target practice zone.... The navy mistakes the bad guys' yacht for the target, & the girls escape in their sailboat... Then we find out the lady with the eyepatch was hiding on their boat... The trick her & push her off the boat...
Then a navy helicopter shows & shots over the PA system that they are in a restricted area..... The girls are somehow able to have a conversation with the pilot by shouting back.....
They open the box with the relic in it, and but it in a jar of fresh water to clean it off.... The relic dissolves, Everyone laments it's loss, and one of the girls pours the jar overboard..,.
The end.....
Sounds like something Jess Franco would make if someone gave him a yacht and a helicoptor?
Maybe "Dykkerne" (2000) ? But it isn't from 80's or 90's and it describers story of two boys, not girls.