April is the hottest month in Guatemala and daytime temperatures reach the mid-80s. Afternoon rainstorms are uncommon in April as it is the dry season, but we recommend you bring a raincoat just in case. There will also be high humidity during the day. Antigua is located in the highlands of Guatemala, so nighttime temperatures do cool down a bit. Prepare for 80 degrees Fahrenheit during the day with 50% humidity and nighttime temperatures around 65 degrees.
The Quetzal is the national currency of Guatemala. 1 USD = ~7.5 Quetzals. Check official exchange rate here: https://wise.com/gb/currency-converter/gtq-to-usd-rate. While it is great to carry cash, almost everywhere besides the markets and street vendors will accept Card/Apple Pay/etc.
Short answer: Yes! Antigua is the most visited place in Guatemala and thus has a strong police presence and focus on public safety. Often when foreign diplomats come to Guatemala, they stay in Antigua rather than the capital because of this. Use common sense and you will have no problem.
No. Hotels/Airbnbs/Hostels will have filtered water readily available and there are corner stores everywhere to buy bottled water.
It is not mandatory as restaurants pay a living wage to their employees. However, if you feel inclined to tip, 10% to 15% is typical, especially in a tourist hotspot like Antigua.