Many plastic surgeons are fully trained general surgeons (I was certified by the American Board of Surgery in 1986) prior to their plastic surgical training and certification, so we can offer proper advice regarding your situation.
If you have fluid under your skin following hernia repair, this could be a seroma, or it could be an indication that your hernia repair has partially or completely broken down, allowing peritoneal fluid (the fluid around your intrabdominal organs) to collect under the skin through what may be a recurrent hernia opening in your abdominal wall.
Difficult hernias are sometimes referred for specialized reconstructive procedures performed by a plastic surgeons (fully-trained ABPS-certified plastic surgeons are experts in reconstructive surgery as well as cosmetic surgery), but this is not elective tummy tuck cosmetic surgery.
Your general surgeon may be trying to deflect responsibility for your present troubles, or may simply be advising referral to a specialist with more surgical tools to attack your problem. It could be that he was simply trying to describe a plastic surgical reconstructive approach in terms he hoped you would understand (since most lay people have heard about tummy tucks), but is really saying he needs help from a reconstructive plastic surgeon consultant in your care. Insurance should cover continued care of your hernia-related problems and additional surgery (by either kind of surgeon--General or Plastic).
Often small or asymptomatic hernias are repaired by a plastic surgeon during the course of an elective cosmetic tummy tuck, but this can and should be part of your original hernia care. Talk to your general surgeon; if he really wants you to see a plastic surgeon, it's not for a "cosmetic" tummy tuck; it's for hernia reconstruction by different means. Also, be careful about any surgeon sticking needles into this "fluid-filled" cavity--it could be bowel!