I replaced the transom in two i/o bowrider style boats. The first one I had a shop do the work for a cost of about $1500. The second I did myself for about $400. First you must remove the engine, drive & all wiring, pumps etc from the inside of the boat. Then physically tear out the old transom from the inside, all rotten wood etc. Leave the fiberglass hull (the part with the gelcoat) intact. On one of my applications I found that the floor was also rotten and had to replace it as well. Depending on the boat you may need to go below the floor and then replace some or all of it. For my installations I replace the transom with a composit material which I bought from a local boat builder for a couple hundred $. You simply cut it to fit into the required space, cut the hole for the dive unit to go through with a rotozip or jig saw. Using fiberglass & resign, fiberglass the transom in place. If you have never used fiberglass before there are several video series available on the web which can be purchased for not to much money. You can order the boat yard resigns and fiberglass mat from west marine or boaters world. Reinstall the engine and you are in business. Not an easy job, but if you are handy, and feel comfortable working with fiberglass you can do it.