Heya! And welcome.
The most distinguishing difference is that Hydraxian Waterlords is an RP-PVE realm, which means that you won’t be able to attack other players usually. Zandalar Tribe is an RP-PVP realm in the meantime, meaning that you can attack other players but other players also can and will certainly attack you.
While as far as I’ve heard on ZT Horde is bigger than Alliance, which is the norm for PvP realms, here as is common for PvE realms Alliance is bigger than Horde, which means more rpers and rp on alliance side also.
The community is active I’d say, events being held, new ones popping up and you can certainly find rp out in the open on a regular basis, you just have to look a bit closer at times and also initiate rp yourself. There’s a (rp) guild thread maintained by Thalei here in the forums, there’s a server Discord that has channels for rp discussions and also for guild advertisement, and rp events are being advertised on both platforms also. Lastly there are respective ingame rp channels also.
If you don’t have it yet I highly recommend either of the addons, Total RP 3 or MyRolePlay 3, to be able to customize your character and to be able to easier see other rpers around you.