Your water heater is probably not going to be the bottleneck unless you take really long showers. A more likely problem is getting your supply pipe big enough. The pressure comes from your utility and the flow rate (ie "pressure" across multiple fixtures) is determined by the pipe size.

Oh, I didn’t think about that, thanks for the info! The one I’m looking at has 4 jets, each doing 1.8 gpm. Do you have any idea what size pipe you would recommend for that?

With 4 body sprays and a regular shower head, you're looking at 3/4" on both hot/cold if you want them to work well. That said, it really depends on your specific setup. Worth consulting with a plumber.

Four jets at 1.8 gpm each? That would be 7.2 gpm, which means you would: 1) run out of hot water quickly, and 2) greatly violate plumbing code in many areas of the country.

After you fix the pressure issue, maybe with adding a separate pressure tank, then you have to fix your hot water supply issue. 7.2 gallons per minute, would mean a 10 minute shower with a typical tank. But, if you had enough pressure, you'd be blasted clean in that 10 minutes! But no one after you would have hot water available. A commercial boiler could possibly create enough volume to sustain multiple showers. That would be very challenging production for a tankless, except in the warmest areas of the country, like Florida. Incoming water temperature controls the temperature rise and flow that is possible. A series of 2 tankless, with the first acting as a preheater, might possibly work for a cold climate. But it also might not work, as they have minimum flow rates to trigger the burner. There is a lot of math to figuring out the possibilities here, and they start with your location and specific water conditions.

7 gallons down the drain is a whole other separate physical plumbing issue. That will require upsizing the drain, or multiple drains. I have not even gotten into the issue of the waste of the resources, which does violate code in many many places that have experienced water shortages. You may be surprised at how many locations in the US have experienced drought conditions and water shortages. It is only getting worse.

Go looking for low water pressue shower towers there are many .

@Trevor's House - just make sure you really want these. I know several people who installed them, with all the required plumbing expense that goes along with them. All of them say that after a few uses the novelty wears off and they don't use them again. If you’ve already tried them, and love them, then disregard my post. But if you have not, then think it through before spending so much effort and money.