Low power and oily water in bilge

I bought a 2002 Catalina 320 in October with a 27 hp inboard diesel. I had a brief overheating problem, but must have been something clogging the sea water intake. Since then I seem to have a power loss (4 knots at 2600 rpm). Bilge runs frequently while engine running and water in it seems oily. I'm new to diesels. Any ideas?

Are you saying that your speed at 2600rpm is down, or that the engine won't make higher rpm's any more? If the speed at 2600rpm's is lower than it used to be, when was the hull last cleaned and the propeller inspected? Reduced speed at the same rpm's is not an engine issue. It could be a gearbox issue or it could be (more likely) a dirty hull or dirty/damaged propeller.
The water in the bilge could be from the prop shaft, a leaky heat exchanger, or a leak in the exhaust elbow or exhaust system. It could also be a head gasket or similar, but less likely. How much oil is the engine using, and is it back running at normal temps, or running a bit hot still? The oil could be from an oil leak, but a leak in the exhaust will put oily water into the bilge and that is quite likely your issue.