Yes... it is possible to do a "Thru-hull" on your Seafarer. But, assuming you have the standard/supplied "transom-mount" ducer, you would need to buy/upgrade to a true "thru-hull" type transducer. This model - (link is only an example, you might find it cheaper elsewhere)
http://www.anchorexpress.com/Garmin-B60 ... 982-00.htm is a good choice. Has dual frequency, temp. and a very low/flush profile. The 20 degree tilt should be right on for a Seafarer. Keep in mind, you won't have water speed, but it's not really an issue because you get that from your GPS anyway. You can probably get an optional speed pick-up, but again... why bother.
You can also do a "shoot-thru" install. The ducer gets epoxied, (or use a good sealant) right to the inside of the hull, (same area as "thru-hull"). The signal literally "shoots-thru" the bottom of the hull. You will lose some power, (signal strength) with this method, (one of the "cons") but it is the "cleanest" set up, because there's nothing mounted outside, (no drilling holes, no exposed wires, etc). You can get-a-way with using the standard/supplied "transom-mount" transducer, or purchase an optional "puck", (shoot-thru) style. However, with this set up, you won't have water temperature or speed, (another "con") unless you buy the optional sensor/pick-ups, and that brings you back to drilling holes in the transom, (sigh) .
Imo, (I agree with the above replies) the most practical set up is the "transom mount", especially since you already had one mounted. Just replace your old one in the same spot. You won't lose any signal strength or give up speed or water temp.
If the hole pattern is different, try to use one or more of the old holes, (if possible, but don't sacrifice a good location) and fill any un-used holes with either "Marine-tex" epoxy -
http://www.marinetex.com/ or use a counter sink ss screw, (counter bore the gel-cote) so the screw head is flush with the gel-cote surface. Use plenty of "under the water line approved" sealant on all screws.
