The water is treated, so it won’t kill you, but the pipes and storage systems are not good, so either take water purification tablets and your own water bottles, or buy bottled water - it’s widely available. I had no problems using tap water for brushing my teeth, nor eating salads.
Not sure what other advice you’re after? If you are travelling independently, book buses to your next destination as soon as you arrive, particularly in Havana. Be prepared to queue a lot, especially if you want to buy a Nauta (internet access) card. Trust the local casa familiares owners to be able to signpost you to services, other accommodation in your next area etc. Watch out for scams, but don’t be paranoid. Having some Spanish, even if it’s not central/southern American Spanish, adds enormously to the experience.
The Sunday art market in Havana is brilliant.
Having said it’s a wonderful country, it’s still very poor - be sensitive about your (comparative) wealth.
Are you travelling independently or on a tour ? . If on a tour you’ll be fine , no need to get anxious. If independent, get an internet card to sort out future stops , buses , taxis etc .your hosts should give lots of info . It’s a lovely country , we felt safe but were careful regarding money , phones and walking around . But then I wouod be the same in London .