Appropriate relay for water heater rated for 220VAC/1500W
Inductive switching can cause extremley high voltages to appear across the contacts. This can cause damage to the contacts, but worse still, it could jump across the pcb and damage your low voltage circuit.

Personally, I wouldn't use either of these to drive a mains load; instead I would use one of these to trigger another relay that is better equipped to deal with mains voltage/current and inductive load switching and is away from your low voltage circuit.

Double pole switching is great for full isolation, but if you are using two  different relays to control this, then you would need to be very careful to check the mains was not live before working on it. For instance, if the relay controlling the phase was stuck on and the neutral relay switched off, the circuit would be broken(off), but would still be live.

This is only my opinion. I recommend that only experienced electricians work with mains electricity.