Thank you for your question. The nuances of every situation are different, so two cases that are exactly the same factually could have entirely different results due to something as simple as a witness' ability to effectively articulate what he saw. There is no usual outcome simply because every case is different. I understand wanting to know what will happen, but the question shouldn't be what will happen--instead, the question should be what can be done to get the optimal outcome possible.
When suspected of committing a crime, the suspect and any cooperating witnesses should immediately terminate all communications with law enforcement and any one else involved. The best way to respond is to either politely decline to discuss it without the presence of counsel, or to simply say nothing at all. It doesn't matter if you are threatened with arrest, prosecution, or anything else, no statements should be made except through an attorney.
The prosecution has a difficult job--they have to prove guilt beyond a reasonable doubt. In your case, the victim claims that she was not being drowned, so that immediately presents a challenge.
Maybe there are two witnesses, and maybe not, but the charges are serious and you don't want to go to prison. If you are possibly facing attempted murder charges, you would be a fool to not acquire a criminal
defense attorney now. A criminal defense attorney could guide you toward preserving the case of your innocence. For example, a criminal defense attorney might recommend that girlfriend is examined by a physician to verify that she does not have injuries consistent with the alleged struggle. You need that legal counsel.
In short, don't talk about your case to anyone, and get a criminal defense attorney immediately.
Let me know if further clarification is needed. Thanks.