i don't know the specifics of your boat but it should be the same basic design on most boats. you will have an intake hose and a return hose to your core which is basically a radiator like in old houses. hot water comes from the cooling system of the motor and goes through the core and air is blown across it and then through your vents. yes, you can swap out the fixed vents for pull out vents. i think skidim has all the parts. you may not be getting any water to the core. the PO on my boat had just connected the 2 lines at the core when the core started to leak. the fan would still blow air across the core, but it wasn't warm as it wasn't getting water. someone may have put a valve in the hose from the motor to the core. this would be beneficial if the system developed a leak under way and you needed a quick way to shut it off. or it may have been winterized and never reconnected and the hoses are capped at the block.
another possibility is maybe your motor isn't hot. i know this sounds silly, but when my thermostat had a piece of the impeller in it, it was stuck way open and the motor wouldn't get above 120 degrees. this yielded only lukewarm air at best out of the heater. once i fixed the t-stat and got up to 160 degrees it was nice and warm again. i do notice that my heat is a little better when i'm underway. this makes sense since the motor is putting off more energy as heat and the water pump is pumping faster. btw this is on an 02 X30 with 2 vents and 1 fan speed.