You will need to open the back because the only way that you have a chance of getting the phone to work properly again is as outlined below.
As you know water and electronics and electricity are not a good mix.
The minerals in the water cause corrosion and the water also provides circuit paths for the electricity which were not in the phone's operating design and could damage the components. The corrosion process starts immediately and will not stop until it has been thoroughly cleaned away.
Rice is good to eat but does nothing to fix the problems caused by corrosion.
Don't Put Your Device in Rice. Here's Why
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 completely dis-assemble the rest of the phone and clean all the affected parts using Isopropyl Alcohol 90%+ to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% or less and is not as effective. If you do check the label to verify the amount
Here is a link that in general 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.
You may also have to replace the battery as sometimes it is hard to recover a battery from water damage.
Here is a link to the Ifixit repair guide for your phone which may be of some help.
Nexus 5X Repair
If this process seems too daunting, take your phone to a reputable, professional mobile 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.