not sure if you catch this in the development phase

-turret deals damage to friendly building, including wall
-the damage of water spray is low is understandable, but steam is imo too low even after damage modifier 5, I think it should be 20 or at least 10, otherwise, there is no reason to use steam as it won't be able to kill anything anyway (alternatively, just put damage modifier setting)
-have you thought about using the tank flamethrower attack pattern instead (if possible)? I think it would serve the purpose of your turret better instead of playing catch up and the biters will still reach your wall anyway (and get damaged by your own turret), also thematically, the turret basically pushed back the biters with water pressure, stopping them in their track

in any case, this is what I'm looking for for a while now, a different kind of early game AoE turret (because shotgun turret suck), so thank you and good luck

-turret deals damage to friendly building, including wall

Also including rails running outside the wall -- but I guess they would also be damaged if they were in an area covered by a flamethrower turret instead of a water turret.

-the damage of water spray is low is understandable, but steam is imo too low even after damage modifier 5, I think it should be 20 or at least 10, otherwise, there is no reason to use steam as it won't be able to kill anything anyway

Not true! There's that one nest of biters I have close to my wall. It's well out of the water turret's reach, but biter groups gather at a position that is barely within reach of the water turret. With an extended cool down time (16 minutes minimum instead of the default 4 minutes), I have seen medium biters being exposed to hot steam from the water turrets long enough to be killed, and I've also seen big biters taking considerable damage in the same location. Yes, it takes a very long time before a water turret can score a kill, but it is not impossible. :-)

What I didn't check yet (away from my Factorio computer at the moment): Does the steam temperature have an effect on damage, or is the damage the same for steam from a normal vanilla boiler and "nuclear steam" (for lack of a better expression, I mean the hotter steam used to power the steam turbines)? I think a damage factor based on steam temperature (don't use fixed temperatures, please, so mods that use steam with a temperature somewhere between the min/max values allowed by the vanilla game would cause different damage) would be nice to have. Could you implement that, please?

-turret deals damage to friendly building, including wall
-the damage of water spray is low is understandable, but steam is imo too low even after damage modifier 5, I think it should be 20 or at least 10, otherwise, there is no reason to use steam as it won't be able to kill anything anyway (alternatively, just put damage modifier setting)

Fixed in the latest versions (0.0.2 + 0.18.1).

-have you thought about using the tank flamethrower attack pattern instead (if possible)?

Perhaps later … perhaps.