Thinking of you Chattanoogachoochoo, I hope you're feeling much better today.
I have been through something similar and wanted to offer you a little insight into what may happen next. I am a fairly regular mumsnettter, but have name changed as I have kept private about what happened to me.
Your P will either be charged or released, probably today in all honesty - I am guessing that if he is charged, it will probably be Common Assault (battery). If he is charged, it is very likely that bail conditions will be imposed - these will centre around him not being able to contact you directly, or through a third party and also involve him not being able to approach your property. These conditions will be imposed for your safety and, if he breaks the conditions, you can inform the Police and they will re-arrest your P. Scenes of Crime officers will come round at some point, either today or tomorrow to take photographic evidence of any bruising/ swelling. Any photographs you may have already taken will not be used by Magistrates or Crown. I don't know if the police informed you of this, but Social Services will be informed of the incident as your DS was present during your assault (and also a victim during the water throwing incident) - what they decide to do obviously depends on a number of factors, but I can tell you to not worry - they are there to help you and you can also contact them, further down the line, about establishing contact for your PIL's and P. I hope that the police also informed you that by giving a statement, you largely lose your right to drop charges. This is because of the shocking number of victims who are coerced into dropping charges by their partners, so the police now have the right to pursue charges without your authority once you have given a statement. They should have also passed your information to the local domestic violence charity who will call you and offer to talk through and listen to what happened. Whether you choose to is entirely up to you, but they can be a valuable source of support in the coming days and weeks. Womens Aid is also an excellent support line and they can offer advice on many issues.
If your P is charged, the outcome of events largely depends on what happens in the coming weeks - his conduct before the magistrates, how he pleads, whether it goes to crown, etc. At the moment, you do not need to think about that. The police and domestic violence unit will be there to support you through the court stages. Please take each day slowly, at the moment you will be feeling angry and possibly somewhat victorious in that you have safely had him arrested and brought to account for what has happened. But, in the coming days and weeks, you will go through many emotions - anger, sadness, grief, disappointment, uncertainty and helplessness just to name a few.
Be kind to yourself - you will have good and bad days, but you have most certainly done the right thing for you and your son.