Should I spend the money to get my water heater diagnosed or just replace it?

1993? It's nothing but rust inside. Not worth having someone come out only to tell you you need a new one. Once a hot water tank starts deteriorating (probably nobody maintained it or replaced the anode rods), there is no way to save it. The fix is a new one.

Just replace it. It's past the usual lifespan of a water heater, and once they start leaking they can't be fixed. There is no point putting off replacing it. I just got my water heater replaced. I bought it from the store myself and brought it home. Then I got a plumber who is licensed and works for a local plumbing company but does extra small jobs evenings and weekends to install the new one.

The heater cost $350 and the plumber cost $60. He told me that if I had had the plumbing company do everything, they would have charged me about $750. Measure carefully if the heater has to go in a small space. New water heaters have more insulation than the older ones and the same capacity heater may take up more room than your old one.

It sounds like the unit is starting to fail; this means that the "bottom" will drop out, spilling the water all over the floor when it rusts through enough. This can happen in the next few days or weeks. There is no practical fix for this; replacement is needed and you have had a good lifetime on the heater. I suggest replacement

Rust? Most tanks are fiberglass, but yours is shot and failing. By the time you notice any water, it's been leaking for a while and soaking the insulation until saturated. Get it repaired before you have water damage, because before long the slow leak is going to burst from the water pressure.

The plumbing industry usually quotes 10 years as the useful life of a heater......I've gotten much longer out of electric heaters.....gas heaters don't last as long .......you're on borrowed time......no way to fix it and it wouldn't be worth it any way.

They cost a lot more than they used to: be prepared to spend $1100 or more installed. Good news: a new tank will probably used at least 30% less energy - your bills will go down.

It's rusting out. There's no way to repair that. Just get a new water heater. Incidentally, you got a LOT of use out of the current one.

You should definitely just replace it. Don't even consider fixing it. It probably can't be fixed anyway. Just replace it. It is way past it's lifespan.