I have a Reinell Bowrider, with a 190 HP Chevy engine, that hasn't been started in about 5 months.
Before starting it, should I change the engine oil? Or will it run okay without changing it?
As far as the oil is concerned, you should be okay. Check oil level and start it.
Other issues may arise with a boat engine (likely with a carburetor) that's been sitting around for 5 months, for example rough idle, etc.