There is a brilliant charity - www.gbss.org.uk who have loads of information on their website.
In my first pregnancy, I had GBS that had not been diagnosed (this was 2021 - i had covid when i was due to sort out the test and then DD1 arrived two weeks early). DD1 developed sepsis at 23hrs old and spent two weeks in NICU. She was very poorly. She had amazing treatment and is now a thriving 3.5yo.
For my second pregnancy, everyone took my GBS status very seriously. I was given a big sticker to put on the front of my notes, antiobiotics when I went into labour and DD2 was monitored for 24hrs after birth. She did not fall ill. One thing to know if that you need a certain amount of time between being given the antiobiotics and baby being born for them to be effective. But if you have a super quick labour, they will just keep you an extra night for monitoring.
You can absolutely still have a water birth with GBS. In fact the charity I linked to had a post about this on their insta feed today.
Whether you need to go to hospital straight away depends on when your water breaks i think. Both times, my water broke before contractions started, which is high risk for infection. I think if you are having contractions first, you can stay at home for a bit but please don't take my word for it - ask your midwife.
Please have a read of the website i linked to and they have a helpline you can call as well - they have some clear information on what to look out for after birth. It all sounds scary, but it's actually a good thing that it is found early as it's awful being in my position and only finding out when it's too late. Something like 40% of women carry GBS and only a very small % of babies fall ill, it is just about making sure you know the signs you should be looking out for.