Waterbed Effect Still Present After Doc Tried to Aspirate Abdomen and Came out Dry. Should I Worry?

Even if you had some old blood, you do not have an actively-enlarging hematoma. This is either old blood and serum, or seroma, both of which require the same treatment--aspiration one or more times, compression, and if not resolved in this fashion, replacement of drain, sclerodesis, and (rarely) re-operation for pseudobursectomy.

After a month, if these tissues haven't "stuck-down" yet, they become progressively less likely to do so, so there is some degree of urgency here. That being said, if your surgeon's partner was unable to hit the seroma, this is understandable since he did not do your surgery and does not know the exact thickness of your subcutaneous fatty layer. Most of the time seromas are easy to tap, but not always! Of course, your surgeon's partner did not want to stir up bleeding or go too deep and hit an intra-abdominal organ! So congratulate him for his caution.

Once your doctor returns, you should see him promptly and have this aspirated. It will be done through a mostly or completely numb area, so you need not be worried about discomfort. But it does need to be drained, especially if there is enough fluid for a waterbed effect. Best wishes!

As long as you're not having fevers or severe abdominal pain, it's okay to wait 5-7 days before attempting another drainage of the fluid.  A low salt diet and compression garments may help in the meantime.  However, in my experience seromas resolve after 4-6 weeks of serial aspiration of the fluid.

Seroma is fluid underneath the skin that can accumulate after tummy tuck.  It usually means that the abdominal skin has not stuck down to the underlying abdominal wall.
Compression and drains help prevent Seroma in the early post operative period.
It sounds at this point as though you may have a persistent Seroma. Frequent drainage is the best way to go.  Sometimes these will resolve with multiple drainages, but on occasion they can require reoperation to get rid of the scar tissue shell that surrounds the fluid.
If it has gotten worse since use saw your doctor's partner then I would ask to go in again and have him look at you once more.  If you think it is better or stable see your surgeon as soon as he returns.
Good luck to you.