Hi @fixzahid ,
Water resistant doesn't necessarily mean waterproof.
The specs state that it can withstand water up to 1 metre for 30 minutes, which is a bit different to 12 hours submerged!
Whilst there are no problems for now that is not to say that they won't appear in the near future.
Just so that you are aware of this
Personally I would open it up to inspect it but I'm just cautious around electronics and water especially if the device is not new.
Are there any minute cracks in the case or flaws in the seal due to normal activity over time which may have compromised its’ water resistant integrity for example?
Here’s a link to the ifixit Samsung Galaxy S5 Mini guide, if you decide to do this
This link may also be of some help. Electronics Water Damage
Hopefully you also cleaned the outside of the phone appropriately given where it was found submerged and also how the phone is used ;-)