Thanks. I just re-checked just to make sure, and still the same,
although I did confirm that it's 240 between both poles. Actually,
fwiw, it's 237, and going from each to the neutral is 118 and 117 (I
think; could've been 118 and 119). But, clearly not zero.

I was measuring at the breaker by touching the screws, but thought it
possible that the wires weren't screwed in tightly. I tried testing
behind the screws, but couldn't get a reading. Am I right that
touching the screws won't necessarily give me what's coming OUT of the
breaker? I wiggled, or tried to, the wires, but didn't note any
looseness, and re-checked at the heater, still with no voltage.

I guess next step is to undo and re-attach the wires at the breaker?
Haven't done that yet, and hesitate to if not needed.

There is no use to worry about a couple fo volts differance.

Cut off the breaker and test to make sure the voltage is really off.
Switch the meter to ohms and see what the resistance is to the heater.
It
should be very low on the two wires and almost an open circuit to the
ground
wire. If the resistance is low on the two wires in the breaker box
comming
from the heater, the breaker must be bad. If it is almost an open
circuit,
go to the heater and measuer the resistance of the wires going to the
breaker. If low, you have a bad wire. If high, the element is probably
open, you can measuer that.

You may also want to make sure the power is off, then hook both hot
wires to
the ground at the water heater. Then go to the breaker box and check
each
wire to ground. If one is low and one is ooen, you have just found your
open wire and will have to trace the wiring and maybe replace or splice
it.

Thanks for the great, specific advice! In exchange, I have a feeling
I'm going to be asking a stupid question.
So, I turned the breakers off (two 30 amp ones), and pulled the wires
out of the breakers to measure the resistance between them. Right? I
get 1. Likewise, when testing from each wire to the neutral bar
holding all the white wires, that earlier I used to measure the 120 v
coming into each main wire to the box.
I went to the heater, and also measured the resistance between the
same wires coming out of the wall, and also got 1. likewise when I
went from one of the wires to a ground (metal part of the heater).

Did I do this right? what does this tell us? Sorry for my ignorance,
and thanks so much for the help.

UPDATE: LOL... okay, very well could be some "user error" in place
here; what a shock, huh? I replaced both breakers, just in case. I get
120 between each and neutral. Went to where the wires came out of the
wall and joined with the wires going into the heater... and realized
that one was white (flesh tone), and the other black. Duh... So,
obviously the black is coming from one of the breakers; not sure
where the other black goes into the heater, but it isn't this
off-white one coming out of the wall.

Still no reading between the black and off-white wires coming from the
wall; 120 between the black and ground; nothing between off-white and
ground.

A Black and Red wire are attached to the top two poles of the upper
thermostat, coming in from above. No voltage between them. This should
read 120 or 240, depending on system, right? I do get 120 between each
pole and ground, though.
Pushing the reset button does nothing, so it apparently wasn't
tripped.
Does the lack of voltage between the two top poles, L1 and L3,
indicate a bad thermostat and it's as simple as that?!

Okay, replaced the upper thermostat. Still no reading between L1 and
L3; should there be? Still 120 from each pole to ground.
No reading between the upper element contact screws, so no power is
getting there. Another bad thermostat? I'm stumped (but that doesn't
take much).



*If I understand you correctly the wires on the circuit breaker are not the
same wires that come out of the wall and connect to the water heater. If
that is the case there may be a junction box somewhere in between and one of
the connections has come loose. It is also possible that someone made some
sort of hokey splice somewhere and that has failed. Are you able to
physically follow the wire as is comes out of the circuit breaker panel?