Duct Tape It

As we all know, duct tape can fix practically anything. So of course it can fix your leaky blow up doll.

Just cut a small piece of tape and apply it firmly over the hole.

It's not the most attractive solution, but duct tape is durable, seals well and will stay put.

Other repair options:
With a Band-Aid: Ideal for smaller tears, can match skin tones of your doll
With a Tire Repair Kit: Most robust solution, if you really care