The only option I know of (short of lifting your boat out of the water), is to quickly insert a plug in the transducer hole. You can do that from the inside if you have a plug and hammer available.
A few liters of water won't sink your boat. Pull the transducer and have the plug ready to insert. Or, If you have a friend who could dive under the boat while you pull the transducer insert a tapered cork, rubber, soft pine, etc plug from outside the hull, you won't get much water in the bilge. Water pressure will keep it in place.
Then, when you have the replacement transducer ready, reverse the process.
Insert transducer element and tap lightly to force out the plug.