Why my phone is stuck on downloading screen?

Hi,

As you know water and electronics and electricity are not a good mix.

The water causes corrosion and provides circuit paths for the electricity which were not in the phone's operating design and could damage the components.

First do not turn on your phone and then you need to with the phone turned off, remove the battery as soon as possible from the phone to minimize further damage.''

Then you need to dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 90%+ (available at most pharmacies), to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is sometimes only 70% IPA or less, can contain scents and other ingredients and is not as effective. If you do check the label to verify the amount of IPA

Here is a link that describes the process.

Electronics Water Damage

As always with electronics, especially surface mounted PCBs be gentle when handling and especially when brushing away the corrosion. You do not want to remove any components from the board.

Hopefully after you have done all this the phone might possibly work correctly again.

Here is a link to the ifixit guide(s) for repairing your phone.

Samsung Galaxy J7

If this process seems too daunting, take your phone to a reputable, professional phone repair service, experienced in water damage repair and ask for a quote for a repair. If you decide to do this, do it sooner than later.