Good advice above, as usual. Get a spare bottom drain, as they all seen to leak if they get used regularly. If you're having this problem often, just a tankless won't be any better. Something is off.
Tankless units want very clean water to last any length of time. Sometimes the city water is junk, worse than well water. Pull the anode rod, see what it looks like. You'll need a 1" socket and help, as it's hard to keep the tank still with a breaker bar and 1" socket on the anode hex. I had to hold back with a strap wrench lengthened with a tow strap once.
Maybe the usage is too high on the existing water heater, and you need a 60 gal?
