Okay, my greatest equipment regret is a SUUNTO DC-9 transmitter. A friend offered to let me dive with his spare computer, which is wireless. All I needed to do was purchase the transmitter. Okay, that's a great idea if you are married.
But, eventually, I had to give the computer back. And, after using the DC-9, although it is indeed VERY COOL, as a noob I struggled with all the bells and whistles. Additionally, the DC-9 computer is incredibly rich for my pocketbook.
So, I purchased as a replacement computer - hosed - and gave him back his DC-9.
Let's see... $500 for the transmitter with less than 7 dives on it (which now sits on my dresser) and $700 for a new computer with compass and quick disconnnect.
I financially regret accepting his offer to use his computer and wish I would have invested the original $500 into the computer I ended up purchasing.