The bottom of my tummy is like a waterbed after a tummy tuck, do I have a seroma? (photo)

Congratulations on having undergone the tummy tuck operation; you are correct in that, you should end up with a very nice long-term outcome. The description is certainly consistent with the presence of a seroma but given that your plastic surgeon has aspirated the area it is possible that you are dealing with fluid in the soft tissues (swelling) as opposed to fluid in between the skin and underlying muscle layers (seroma).  Important will be continued close follow-up with your plastic surgeon who may or may not wish to repeat aspiration attempts, depending on your physical examination findings. Best wishes.

Your description is that of a post operative seroma, but very hard to see in the posted photos. Best to seek immediate IN PESON drainage from your surgeon before things become serious or dangerous. BTW did you have drains??? 

It is normal after any operation for your body to produce
fluid. In the tummy tuck operation, a drain is left for a week or
two. When your body slows the fluid production down and the healing
occurs, your drains will stop producing this normal fluid. Typically, the
surgeon will remove the drains after a week or two when the fluid reaches less
than 30 millimeters per day. I give patient's a flow sheet to record the
fluid output on.

In about 1% of tummy tucks, after the drain is removed, the body
produces too much fluid and the fluid starts accumulating. This is called
a seroma and this extra fluid needs to be drained.

Fortunately for the tummy tuck patient, the drainage procedure
is typically simple. A small needle is inserted into the collection, and
the fluid is gently aspirated. Seromas typically disappear after anywhere
between 2-5 aspirations.

Seromas do not typically affect the final outcome of the
surgery, but do require several more visits to the office to remove this extra
fluid.

Compression after aspiration of the seroma is recommended.

The seroma fluid will appear blood tinged in the first few weeks
after surgery, or will be clear if the seroma occurs later in the course.
This is an important distinction between a seroma, which is a fluid
accumulation, and a hemotoma, which is a blood collection. Both are
treatable, but a hematoma is a little harder to aspirate and may have a larger
risk of infection because bacteria grows better in blood than in the seroma.