The oddest I can't realy say (probably a used condom or so), but I can tell you what pissed me off the most.
Car batteries, spark plugs, empty paint cans and other chemicals, rubble from construction sites, drained motor oil in bottles, plastic bags and other household waste, end so on end so on. I worked as an instructor in Malta once, and I've seen it all and on every dive! And then the Maltese are stupid enough to wonder why there's almost no seahorses in their sea anymore, and why there are no dolphins around Malta anymore, and why...
I have never dived in a country that polutes his sea so much, and does almost nothing to prevent it from happening. Unless you like wreck diving I don't think you'll enjoy diving in Malta. There's not so much wildlife anymore in their sea. And to know that they're an EU member state, one should expect more from such a country.