Dave, if the maker of the new gear is certain it will fit the existing thru-hull (and you'll be luckier than most if anyone will guarantee that fit<G>) then I would also make sure to buy it from a vendor who will agree, up front with no doubts, that you can RETURN IT if it does not fit.
Then take it to the boat, try to swap senders, if it does not fit...no big deal. As Gord says, it won't be as much water as you think it is.<G> Yes, you can plug the hole from the outside with a plunger, a tupperware bowl, or a ball of beeswax (Sold as a toilet bowl floor seal, cheaply) if you want to stop/slow the water to a trickle. The beeswax pulls free cleanly, plugs neatly, makes a simple solution. But the time it should take you to pull your transducer, swap in a new one, see if it fits/not and if necessary swap the old one back...shouldn't be a problem. A gallon or two, and even if you let in three or five gallons, that's not going to sink most boats.<G>