Hi Liam,
These fuel filters are most often specific for each boat, not for engine, depending where the manufacturer has decided to put them and which one. I have one V-504-M, but in a different boat, so not of much help to you. Since I have installed it myself, I do not have inline filter. I am not sure that would even be good idea due to very high fuel flow on that engine (at least I have never seen inline filters for such applications). Maybe you do think of that metal mesh inside the fuel pump, but that would be wiser for you to ask one of elderly Cummins engineers, since it is a part of fuel pump assembly.
What you can check, if you already haven't, if the fuel is reaching the fuel pump at all. At the inlet pipe of fuel pump (on the top of engine), there is a bolt. Put some towel below that (it is a bit tight) and unscrew the bolt, if the fuel is present it will flow out. If not, then your line or filters are clogged (or there is some valve somewhere). Of course, even if there is fuel, it does not mean that your filters are clean, but at least, you will know if the fuel is present at fuel pump. And I would do this anyway if it has not been started for a long time, or your filters have been changed, to purge the air from the system. After that, a bit longer cranking is needed, until the pump fill the whole fuel system. (Do this partialy, 30 seconds cranking, 2-5 minutes pause, for the sake of the battery, but if not making it after 10 attempts maximum, than you still have a problem with something). Hope this helps a bit, but I presume that you have already tried this.
Regards,
Nikola