I agree with all of my colleagues who tell you to check with your surgeon, since there are many different answers depending on your surgeon's technique, suture choices, use of drains (or not), and the exact operation you had. But let's give a few general recommendations:
If drains are in place, anything other than sponge bathing is generally not allowed. Drains let blood and serum come out from beneath the skin flaps, but we don't want them to be conduits for bacteria to enter the body. Keep the drains sites clean, dry, and follow your surgeon's recommendations about type and amount of antibiotic ointment or other drain site care.
When your drains are out (or if you don't have them) and your surgeon OKs it, showering is allowed. Gentle scrubbing and not staying in the shower too long are recommended. NO Calgon "take-me-away" marathon sessions--get in and out!
Don't use too-hot water, since your abdominal skin is numb and you don't have your usual protective sensation--blisters can happen easier than you think. Tepid, NOT hot!
No sitting in bathwater in the bathtub. Let's just say that there are a couple of areas chock-full of bacteria that you don't really want to contaminate your water with, since these can enter tiny openings in your incision. I don't recommend bathing or swimming, etc. for 3 weeks after PERFECT healing, longer if there are areas or incomplete healing. This is one way infections occur!
Get in and out of a tub shower carefully--slipping and falling could disrupt your mucle repair or incision!
Call your surgeon or nursing staff first--they really do have an answer to this very common question! Best wishes!