OK here's a vote for the OB
I sail a Catalina 27 with a 15hp OB in the lazarette which is open thru the transom, as from the factory. The 15 gives me about 5.7 knots at 3/4 throttle (hull speed is 6.2). It has electric start and remote controls allowing you to adjust throttle/shift from the cockpit. It gives me a lot of the advantages of an IB, while none of the complications. It's quiet, as the OB is under the lazarette hatch, much quieter than a diesel Cat 30 where the motor is in the cabin. While I do most of the service myself, it takes about 1/2 hour to disconnect the remote cables and pull it out to take to the shop. The OB tilts up out of the water, so I don't have the drag or corrosion issues. I've used this setup for about 26 years and get about 10 years of very reliable use out of a motor before it starts being cranky and I replace it. I can buy a brand new one for about $2K. (The only problem with "buying a new one" is that it has to be a 2-stroke to fit in the well without surgery to the lazarette.) The OB is a long shaft, and since it's in the lazarette and not on a bracket, the prop NEVER comes out of the water when running, even is very choppy seas. The lazarette has room for 2 Tempo 7.5 gal tanks, giving me about 14-18 hours motoring depending on the speed. I used to have a 9.9 which was great as well but didn't push the boat as well thru choppy water and headwinds, but was easier on gas.And if I'm away from home and for some reason the 15hp won't run (it happened once), the 6hp dinghy OB that I have mounted on a bracket on the stern rail will push the boat at about 4.5-5 knots, runs on the same fuel and uses the same fuel line connectors.The electric start OB has an alternator that puts out about 5 amps. enough to run the running lights and instruments and put some juicce back into the battery. And even thought it's electric start, I can pull start it if the battery craps out.I sail in the California Delta (fresh water), SF Bay and the NorCal coast, from Monterey (90 miles south of San Francisco) to Bodega Bay (50 miles north).