The biggest problem is the 500,000 deposit. That is 136,000 USD and most people don't have that kind of money lying about. I can understand people being reluctant to sell up properties in their home countries just to buy somewhere in Dubai when it will never be a permanent home for them. You also have to factor in the rocketing maintenance charges in the marina which can add another 20,000 Dhs a year to your figures, and the housing "fee" which would add another 10,000 or so. Not forgetting DEWA bills (which actually wouldn't be that high for the property you have described.
You could make it work, but only with a very large deposit.