So its was given by an ex that you hate, so what? There's many ways to get rid of the thing if you hate it so much and don't want to be reminded of them.
Girls, don't be such a drama queen, sell it! You get to earn something back from the disastrous relationship. OR you can simply give him back the thing. For guys, oh please, you're the one that bought it, don't you feel wasted that you throw something that cost you so much into the sea? Be a dear and give it to someone else who deserve or want it. I'm sure your sisters or cousins or friends wouldn't mind to be given a free bling even if it was second hand. I know i don't mind. OR give it to the beggers or homeless people if you really think the piece of jewel is nothing to your pocket- ever heard of "A man's trash is another man's treasure"? Imagine what they can do with the big amount of cash. You could be saving a life.
I know some people see their money as dirt but be practical. Don't be a bitch and think of what you can do with the bling. If everyone throw valuables that brings bad memories into the river or sea, you got plenty of people scouring the sea bed now hoping to earn a windfall.
If it's like some of those tragic situations when your loved one's died at sea and their body are never found then you can do the dramatic throw. In this case people would have understood.
