Hi Steve,
I've scraped 2 boats...and the weapon of choice is a 3/4 inch wood chisel, to be used just after going ashore, when the antifoul is all crumbly still. If you wait, it still works, but is much harder work.
The technique to use is to make one long straight clean strip on one side of your chosen area, then using half the width of the chisel blade, take off new rows, so your cleaned area gets progressively wider. Make sure each strip is clean, and then there's no bumps to make the chisel jump.
Once the whole required area is clean, thinners and rags will remove the rest. I'd then cut the hole needed, then use an orbital sander to key the gel coat, ready for primer.
Typically, you'll remove an area slightly larger than the transducer you're fitting, and make good with underwater epoxy and suitable sealants.
The bare gel will need priming, before applying antifoul to the hull again.
There's some specialist antifouls out there now specially for transducers, but they come in small pots and are incredibly expensive.
Mike