How to heat a bathtub when the water heater isn't doing enough?
What about good-ol-fashioned heating or even boiling water on the stove? You'll need a thermometer to ensure your water doesn't go over 120 degrees.
When I was working on my water heater recently, I used spare aquarium heaters to heat up 5 gallon buckets. You can buy up to 300w on those....some aquarists guide will say what wattage heater for say, a 60 gallon or a 100 gallon tank. You can calculate the number of gallons you intend to heat in the tub before you buy any heater.