Why on earth do you want to paint it?
Personally I’d board it out then paint it if at all. I’d be concerned about damp lifting the paint and it looking patch. At least bare brick looks intentional.
I think you’ve got most of the answers above.
Watering paint down really depends on where you start from. There are people (like me) that spend time getting the consistency of paint right so the balance between application, thixotropy and final appearance is as good as it can be.
I’d say it won’t need much water. You’ll be trying to get it to look like a thick “wash”. You don’t want to go so far as for it to look the same consistency as water but I’m struggling to think of anything to compare it to. My suggestion would be not a lot. Get a separate mixing pot take some paint out. Add a little water mix and see how it goes. You want something to wet the surface easily and soak in but not just drip everywhere. Experimentation is the key.
Prep is key so make sure the surface is free of dust, grease and dirt. Check the temperature (humidity, dew point, and substrate temperature 😉 ) as a lot of paints, particularly water based don’t like <10C. I’m holding off on a few jobs until it warms up.