Calculating Electrical Demand for New Construction with Electric Tankless Water Heater
200A Service is not nearly enough
When provisioning new service, by code, you're only allowed to plan to use 80% of the installed capacity.  On a 200A service, that's 160A, and when you deduct your 140A water heater from that, that leaves only 20A for the entire rest of the house, which isn't nearly enough.
Load calculations aside, you won't even be able to make dinner with only 200A.  Assuming either the AC or heat is on drawing 30A, and you have something in the oven, drawing 50A or so there, and then you turn on the hot water at the sink to rinse a dish, the water heater will draw 140A, for a total of 220A -- which trips the house's main breaker.
For a tankless hot water heater of this size, in a house with other standard loads, 400A service (sometimes called Class 320) is required, no way around it.
Or, use a water heater with a tank
A standard tanked resistive water heater only needs a 30A circuit, because it doesn't need to be able to heat the water as fast as it's used -- it can heat more slowly since it has a reservoir of hot water to supply for immediate needs.  This should likely fit in your 200A service without much issue.
If you want or need to draw even less power, then a heat pump water heater is the way to go.  They cost roughly 2-4x the price of a resistance heater upfront, but they use 3-4x less energy, so they tend to pay for themselves quickly -- if the same person buying the water heater is paying for electricity, which it sounds like is not the case for you.  But they also use less power, so if you're trying to fit everything in without needing a service upgrade, it's still worth considering.
Tanked water heaters can also be very useful when your cost of power varies throughout the day, either because you have free power coming in from solar panels, or because your utility charges higher rates during peak hours (which SMUD does).  Newer 'smart' tanked water heaters, (most commonly found on the heat pump kind), are aware of this, and will attempt to heat the water when electricity is cheapest, and save it for when you need it later, which tankless cannot do.
Another advantage of a tanked system in California is that during our increasingly frequent rolling blackouts, you'll still have hot water for a while when the power is out.  With a tankless, when the power is out, you're taking cold showers until it comes back on.
Consider planning for both options
You mentioned in a comment that you're planning to rent this place out, and as such the long term electricity costs are not a huge concern to you now -- but consider that you might want to sell the place someday, and your buyer may take a different view.  Since this is new construction, there are some things you can do very cheaply now that would be extremely expensive later, so it's worth thinking a little about future-proofing.  One of those things that's easy to add now and much harder later is a site for a hot water tank.
As of today, electric heat pump water heaters are the most efficient thing on the market, and are becoming increasingly popular.  Given the challenges the California electrical grid has had over the last few years, I would not be at all surprised if they become required at some point in the future.  IMO it would be a mistake to build a house today that was not compatible with one, even if you initially install something else.
On the other hand, if you do put in a tank system today, to easily let the next guy switch to tankless if they want (assuming they get the service upgrade), you can simply install an oversized conduit from the water heater to the electrical panel -- it'll only cost a few tens of dollars extra.