help determining amp service to my house
A lot of what I am about to answer is personal opinion so there may be quite a bit of comment on this answer. 

It appears that you have a 200A service being fed to a Zinsco Panel which is being used as a service panel utilizing the 6 switch disconnect rule, NEC Article 230.71 (A). The 100A breaker you are referring to appears to be servicing the subpanel.

The Bad news:
You cannot by code add a new disconnect to your main panel. 

The panel is a Zinsco and it appears to be in bad shape. Also Zinsco's are no longer manufactured and are considered to be the second worst panel in the industry (FPE being the worst). They have a tendency to overheat and damage the bus and breakers. I would not recommend putting a high stress load like a tankless heater on them.

Recommendation: Replace your main panel with and install a 200A main breaker. Then I would add a second main breaker for the tankless water heater which would keep it from creating extra stress on the existing electrical system. If possible in your jurisdiction you might upgrade the entire service to a 320A service. You will need to contact your utility provider to check availability.

Final comment: In respect to your question about the 100A breaker. With different voltages (120 vs 240) it's not about amperages but available power. In other words a 120V 100A circuit would be 12,000VA on a single phase, and a 240V 100A circuit would be 24,000VA or 12,000VA per phase. That should clear things up or just muddy the water a little bit more. 

Hope this helps and stay safe.