In my opinion, the rectal route of administration in regards to heroin is the best alternative to using a needle. This ROA has a high bioavailability and the high comes on quick, sometimes providing a little rush. I would highly recommend it as I use this ROA for a variety of substances and have always had good results.
Now, in terms of water, you really only need to mix the dope with enough water to dissolve it. You don't want to make a paste, you want to make a solution with the same consistency as water itself. I would say 5ml is probably enough, depending on how much dope you're using.
Once you have mixed the solution up and sucked it up with your syringe, just lube it up and stick it a few cm's in and slowly inject. Make sure you lay on your side or on your stomach while doing this for at least a few minutes to ensure you don't lose and solution. The effects should come on rather quickly at this point.
PS: if you don't have an oral syringe, you can pick one up at any pharmacy. Some will give them out for free - if you say you bought medicine and don't know how to measure it, blah blah blah - but if you can't pick one up for free, you should be able to get one for real cheap, at most $5. Good luck