So you already have an epay.bg account - that's good...
Sorry for checking the obvious, but you are using the epay.bg website in English, aren't you?
Right - so you are on the epay.bg website. Look top left and you will see a menu item "Payment of bills". Click it then from the drop-down menu, under Utility Bills', click 'Registration'.
On the next page that opens, either search to find your water company by using the search box, or click ''Water Supply and Sewage Companies', then choose your water company from the selection offered. This will put their name and details into the 'Merchant' box on the next page that opens up..
At that next page that opens you also need to input:-
1) your customer number (Абонатен номер), which should be written on the bill somewhere, probably near the top, where your name and address are written.
2) Write a short description of what the payments are for (ie 'Water Bills - House) and make sure the box marked "I wish to get notifications for received bills.' is ticked. (It should be ticked by default, but tick it by clicking in the box if not).
Then click 'add' - and the water account for epay.bg should be set up.
Now a bit I'm not sure about..... I can't guarantee it but I think it's most likely result that if you go back to the website homepage and click 'Pending Bills', then your water bill details should appear and you can pay it.
The other possibility (much less likely, I think) is that epay.bg doesn't recognise bills which were pending before the water account was added, in which case you will need to find a friend or neighbour who can help you out with their physical presence in BG.
Fingers crossed though - and good luck!