Just for interest --- if you end up here in Bangkok and are really 'pissed off' with all the cretinous farang firing their giant water cannons, make your way to Silom Soi 4 (Sala Daeng skytrain exit 1?), which will be water free (security at the head of the soi to confiscate guns).
These days are the busiest 3 days of the year in soi 4, it will be packed from 7ish till closing, continuously!
My choice is Balcony Pub or Connections.
They open for alcohol at 17:00 and are there, dry, till late.
Oh, and forget stories that it all finishes (the water throwing, at 18:00 or 21:00). This is Thailand (T.I.T.) and rules are meant to be broken.
Me? I'll be locked in my room, stocked with Leo Beer.
Oh and another VERY important point ---- do not take money, jewellery, passports etc out with you. Pickpockets abound.
Everywhere, you will see plastic pouches for sale (about 100 Baht) to keep dry any personal items needed to be kept dry. But they are not thief proof.
Finally, have fun.