Can I use 25 amps circuit for number 12 wire if I have on it Water heater (1500 watts) and small pump

No, you can't use light wire for 25 amp breaker. 14 gauge for 15 amp, 12 gauge for 20 amps. Copy and keep this chart, do not use lighter, go heavier if anything. Wire sizes, gauges, for breakers I would also double check that water heater demand. If its an electric heater they are more around 3800 watts

I agree with Ballenger.