Tankless HWH draw a lot of amperage, but much depends on how much electricity is being used by other things in your home. Electric dryer or stove, central air and electric heat all use up a great deal of power. Typically a tankless HWH requires it's own 40A/220v circuit, so that's 40A that can't be drawn on by anything else.
A 200A panel upgrade isn't a bad idea, and I'd imagine it was recommended because there's not much spare amperage left in your existing system. For basic examples, you could visit the Takagi website, they're a major mfr of such HWH and list all the specifications for them.