Re: cabin heater 1988 sea ray 268
Is this boat new to you are has the heater now failed to heat?
If someone had installed a heater using engine water then check in the engine bay for water hoses to and from the engine going forward, often a valve is installed to shut off water flow during warmer weather to prevent cabin heating. make sure any valves are open, if equiped with ball type 1/4 turn the handle should be in line with the hose to indicate flow. Round or T handle may be a gate valve, open CCW.
Hopefully your motor is freshwater cooled, it can be a pain to winterize a raw water system with a heater installed, the heater is often NOT drained/flushed well enough and can freeze.