It's possibly just a dodgy electrical connection, and fixing those - while boring and annoying - is also dirt cheap.
First find the negative lead (call it "earth") from the battery to the engine and make sure BOTH ends are on tight and they're clean.
Then find the earth for the ECU and clean it. Unplug the ECU and plug it back in again a couple of times to get all the contacts cleaned.
Open the fuse box and start pulling fuses out, reinsert them - repeat a couple of times to make sure any oxidisation film has been removed.
Now start doing relays (although I think the D22 might not use relays for those components, but when you're on a good thing stick to it!).
Finally, do the connectors on each injector, then the sensors (MAPS too, go on, might as well).
Once you're done, there's one thing you'll know for certain: it's not going to be an electrical connection issue.
If that didn't fix the problem then it's possible that your water sensor or fuel filter housing might be broken, but at least at this point you wouldn't have spent any cash, especially if you later find out that it wasn't a water sensor but just a dirty electrical connection!
