I accidentally dropped the phone of a girl in the unflushed toilet
Did you manage to get it back?
I have tried everything but the phone seems to be stuck inside.
Well, in the future I'd suggest you not have someone else's phone out while using the toilet. You should learn to be a bit more careful with things that don't belong to you. You should also be honest and tell them what happened before they find out.
Just water damage, right?
Water damage is still bad and can ruin a phone. Offer to pay for the phone if is broken beyond repair.