I'm not sure what size welder you want to run but I doubt that the water heater line is more than 10 ga. You will probably need 8 or 6 ga. Also I am unsure what happens when you put a 20-30 amp water heater on a 40-50 amp breaker.
i'm NOT an electrician.
the water heater is probably 12 or 14 gauge, not even close to big enough. and the breaker is probably a 10 or 15 amp 240v 2 pole.
the basic bottom line is that the whole system, from the main breaker on your main distribution panel, cannot 'step up'. if your water heater breaker is 15 amp 240v, then that is all you will get from that circuit... if your dryer plug is 30 amp 240v, then that is all you will get.
i know from experience that you can run a miller 252 plenty hot on a 30 amp 240v circuit... going to a 10 or 15 amp sounds a little dicey.
btw, the power at my place runs about 117-118v/234-236v. the distance from the pole to the main panel is about 100'.
ampacity and voltage drop charts are readily available online, that's probably where you should be looking.
i'm assuming you have a meter to measure actual voltage on your system? voltage drop is an important factor.
how to get 120v AND 240v from the same breaker without a sub panel to split the 240v through a single pole breaker is beyond my knowledge.
for all other 220v circuits im fairly shure your not allowed more than one device on it. ie you need a dedicated run to the welder or other 220v device.
i have 2 240v plugs running from the same feed, one runs the plasma cutter/welder, the other is for the compressor. they will run at the same time. i imagine the welder turned up all the way and welding while the compressor is running might trip the breaker, but it hasn't happened yet. either way, the wire is heavier rated than the breaker and the voltage drop is within the specified range, so i'm not worried about it.