Tankless Water Heater for Tub

Is it plausible to get a tankless water heater for a tub? We have a 50 gallon water heater and we never seem to have enough hot water for our tub. The tub holds ~90 gallons and it's not like we fill it with steaming hot water. The water heater is new so...

Is it possible to hook up a tankless water heater in series with the regular water heater as an assist?

Any other advice besides taking a cold bath?

Thanks,
BlackNoir