STOP RIGHT NOW and cut 2 of the breakers to the water heater so only 1 of the 3 breakers is switched on. This can be revisited soon after the Load Calcs are complete.
Your panel is screaming out a warning because it's trying to set your house on fire. This is what happens when you add random crud to a service without doing a NEC Article 220 Load Calculation. This is your new term of the day. You get to learn all about these and you'll be doing 2 of them - first on the house's entire service (all loads fed by that meter) to see whether all that stuff will fit on your service... and then another one on the subpanel loads to determine the required size for the subpanel and the feeder wires to it.
So Load Calculation #1. All loads in the house. This is to determine whether which size of electric service you are required to have to support the loads you are running. If your service isn't big enough you'll need to remove loads, order a bigger service, or depend on a 3rd trick we'll cover at the end.
Load Calculation #2. All things which are on the subpanel. If the subpanel powers general receptacle and lighting loads, you count the square footage of the house which it is serving.
If the loads you describe are the only thing on the sub, then I get 112.5A x 100% (not a continuous load) + 8A (2hp motor per NEC) x 125% giving 122.5 amps.
That's a lucky break. It shaves just under 125A, which is a common breaker size, unfortunately it just misses 115A and 120A, which are the amps of #2 copper or #1/0 aluminum feeder. So you will need #1 copper or #2/0 aluminum feeder. Fortunately 2/0 aluminum is a commodity product widely sold. If you're inclined to throw money at safety, buy aluminum wire and a torque wrench - not copper wire. Panel lugs are made of aluminum and science proves torque matters, even on the small stuff.
For now, if your Load Calculation says you can run 40A of water heater, switch off all the breakers except one on your tankless. That will give you a 40A tankless and you will need to get a super low flow showerhead; because it won't support full-temperature hot water more than about 1 to 1.5 GPM (depending on water input temperature). This will require family education but will work you around the problem long enough to get your service upgrade ordered. If your Load Calc says you're good to 80A, then switch off 1 breaker making it an 80A tankless. If you are able to run permanently like that, then permanently disconnect the unused circuit so somebody doesn't turn the breaker back on!
Energy management systems to the rescue!
All this can fit on your existing 200A service, if you deploy some new technology to manage panel load. Unfortunately this stuff is just on the cusp of coming out, and isn't really commodified yet - you can do it with a SPAN panel if cost is no object, but cheap solutions are not really available yet - except for EV charging, they are. So the Hummer is safe lol.
Technology Connections just started a video series on that - let me cue up the part that is most relevant. The jokes at the beginning are worth winding back to the start, though.
Like I say, this technology is just on the cusp of consumerization. But the general idea is that a lot of loads are interruptible - they need to run, but they don't need to run right now. If a dryer gets paused for 20 minutes, that's fine. If a house heat pump or A/C gets paused for 20 minutes, your house will lose, what, 1 degree F in that time? Because effectively, your house is a thermal battery.
So we can solve panel overload problems like this just by interrupting loads which are interruptible. Tankless heaters and ranges are non-interruptible, then you have a pecking order of priorities: dryer > HVAC > tanked water heater > EV charging. Notice how all those are storage loads - they need to do it, but not right now. Drop loads off the bottom until the load is within service capacity.
When a load has this type of interruption or throttle-back, it disappears entirely from the Load Calculation.
The SPAN panel will do this right now if cost is no object. Otherwise, you'll need some load interrupting devices to knock out lower priority loads when the heater is on.
You still need to enlarge the feeder to the subpanel, and the subpanel, and the supply breaker to it.