I want to put a
tankless water heater in my house but I want to make sure the panels
can support it.

It's the AR115 http://www.boschhotwater.com/BoschHotWatercomHome/ElectricProducts/PowerStarAE115AE125/FeaturesandSpecs/tabid/397/Default.aspx

requires 80amps and 2x40amp breakers 240/208v

Pictures below
the picture with the large panel is the outside main, I believe there
is a 90amp feeding the panel in the garage which is the smaller on in
the pictures. The 30amp breaker in the garage panel is the dryer and
the other smaller 20's are just receptacles but I think they went a
little over kill with one receptacle per breaker. I think the overhead
lights run off one of those as well.

My question is can I install the water heater safely into this existing sub-panel?

[IMG]www.orlandotechpros.com/heater/main.jpg[/IMG]
[IMG]www.orlandotechpros.com/heater/mainclose.jpg[/IMG]
[IMG]www.orlandotechpros.com/heater/maincovered.jpg[/IMG]
[IMG]www.orlandotechpros.com/heater/mainout.jpg[/IMG]
[IMG]www.orlandotechpros.com/heater/sub.jpg[/IMG]

http://www.orlandotechpros.com/heater/main.jpg
http://www.orlandotechpros.com/heater/mainclose.jpg
http://www.orlandotechpros.com/heater/maincovered.jpg
http://www.orlandotechpros.com/heater/mainout.jpg
http://www.orlandotechpros.com/heater/sub.jpg

NO WAY you have enough power to do this. ESPECIALLY with electric heat in the house.
You'll likely need an upgrade to 320/400A to make this happen.

Those whole house on demand units are power HOGS for short terms. IMO they are simply NOT worth the trouble.

I don't have electric heat, I have a heat pump with emergency electric back up that hasn't been used. The on demand is useful here in Florida where our water is 72 our of the ground year around and I'm in a 1.5 bath house with only 2 people.

That is still electric heat. Knowing that this is Fla would have helped a bit from the start, but it really does not change much. Fact is, the heat can and will come on at the same time at some point. If not then why have the heat at all?

I think the only way is to have a qualified electrician do a load calculation on your house to see if your service is enough.
I will also add that I personally would not want to put that much of a strain on that old Zinsco panel. They are notorious for failure and burned busses.

Also there are a few breakers no in use anymore in the panel. A 2 pole 30 amp at the top, the 2 pole 50 amp, and a spare 2 pole 15 amp at the bottom. The 100 amp feeding the AC/Heat should now be a 60 amp after replacing the old Unit last year.

Don't you think these are details that would have helped right from the beginning?
What else is there that we don't know?

This is why the system needs a survey by a qualified electrician to perform the proper load calculations.

All actual loads of specific appliances must be collected, along with the square footage of all livable space, must be calculated using the proper derating factors as allowed by Code.

Adding almost 72 amps to a 200 amp service that will only allow 160 amps max, and considering the apparent load is too close to call here.

Also consider how tired that existing panel appears, 72 amps will be a great stress on that system, even if it does fit.

From personal experience... I would NEVER install an electric tankless water heater in a residential setting, and rarely in a commercial setting.. they are energy hogs, not energy savers.