Auxiliary water heater?
 

This is an odd question but: I like taking hot baths every once in a while, but our current tank isn't big enough to fill our tub. I don't really want to replace the tank because it's plenty big for all other uses; I can take a shower, wash dishes, run laundry, etc. with no problem but I just can't get a good hot bath. I'd hate to spend money to replace a perfectly good heater, and I'd really hate wasting energy heating more water than I generally will use.

Has any one heard of something that could just temporarily heat a larger volume of water and can be turned off when not needed? (Other than a kettle, smart-arses. ;))

Tankless water heater.

Hello, this might help you. http://en.wikipedia.org/wiki/Water_heating#How_tankless_water_heaters_work
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Probably your wh is failing but you can check it.

With incoming water at 50F and a 30 gal., 110F bath, your wh settings and gals needed are
110F, 30gals
120, 26
140, 20
and if you boil water in a kettle it needs to be 11 gals, a pretty big kettle.

A tankless water heater would cost in the rage of replacing your existing water heater and you'd have to install it somewhere close to the tub (do you have a space available?) - so sure it would work but not save you money.

Some other options are: get a hot tub (they have their own heater) or get a second water heater tank and have it plumbed so it can be shut of and water will not flow through it.

None of these options are very cheap - probably the cheapest option is to replace your existing hot water heater with a bigger one because of the minimal plumbing changes.