I used to scuba dive (PADI instructor at one point, in Hawaii)... I don't know much about commercial diving... all I can think of is "detects sulfur dioxide" or "detects traces of petroleum" or "detects Laser Jellyfish" or "detects killer Jellyfish Laser Beams" (oh wait... that's already been suggested and you said "no").
... maybe used in taking underwater soil (mud) samples? Used in straining / filtering surface material from ocean / sea bottoms?
... from the appearance ... maybe taking algae, plankton, or seaweed samples? (different sized holes, and you said "different shapes for different applications").
(I know there probably is, or should be, an unwritten rule for the thread, "one guess per customer per post", but I figure after so many guesses by different people, it is a "hard" object, so some leeway is called for)
Okay. Here is a hard one, then. BUT, it was reported in the news.
This caused a lot of problems for Amateur Radio operators, and OTHER people in radio communications (incl. professionals), off and on from the mid 1970's to the late 1980's.
(I actually worked on the "case"... so to speak... that is, it impacted on my job, and we helped to analyze and characterize the thing).
It has a nickname... two words... a nationality and a bird.
However, if a SPECIFIC answer isn't guessed / found by anyone, I'll answer for a good GENERAL guess. I've already provided a clue (" ... caused a lot of problems ... "), but no, it is NOT a jammer.
And no, it does not detect UFO's/Flying Saucers, or evil laser jellyfish.