Can I replace a 30 amp breaker on my panel with a 25 amp for my new hot water heater?
Read the instructions carefully.   30A is an industry-standard convention in North America for tanked water heater circuits.  The NEC derating rules require you to provision service for 125% of the actual water heater draw; water heater manufactureres know this, and size their water heaters to take about 23 amps. (Which derates to just under 30 amps, voilà). 

Underwriter's Laboratories (who pretty much writes the book on what is acceptable, and harmonizes that book with NEC), goes in lock step with this.  They certify the labeling and instructions as part of the UL listing (which is why instructions are often so bizarre, it's because they are copy-pasting instructions UL has approved in the past).  So I would expect approved instructions to state 23-ish and require a 30A breaker.  

Because 30A circuits are so standard, and 25A breakes such an odd duck (and difficult to find, encouraging misuse of alien breakers), I think UL would expect poor compliance with any "change the breaker to 25A" instructions, and would push back on the water heater manufacturer to make the heater safe with a 30A breaker.  

The only advantage I can see to a 25A breaker is if you are using 75C rated wires (not NM/Romex!!!), you could wire it with 12 AWG instead of 10 AWG.  But that would encourage another badness: up-breakering 12 AWG to 30A when the owner gets any other water heater on the market that of course requires 30A. 

Any existing 30A circuit should be 10 AWG wire, and 25A is well within its limits.  So changing the breaker will suffice.