Very little fuel will come out. Place a wad of paper towel underneath. However, if you have an f-04 code it sounds like you may have a coke buildup. This usually happens when the heater is turned off and has not run hot enough or long enough. It is good practice on a regular basis to put the heater on high and let it run a bit and then turn it off.
Before checking and replacing the filter I would clear the code by resetting the unit and try staring it again. If it starts, let it run on high for a while. If you replace the filter, try to add fuel to it before attaching it. It could take several tries to get the fuel primed out of air to start the unit. The unit will lock out after four attempts and then you have to do a reset. This could take several times.
Also, how is you fuel level in the boat? The pickup for the heater is usually quite a bit off the bottom. My experience has been that around 40 gallons remaining I would lose prime and get an error.