When you say it was in water, did it fall in, did water flow over it, how did it get it get wet?
If it did drop or fall, then perhaps the water did not cause the problem.
Logic dictates that if it was working properly before it got wet and if it did not drop or fall into the water, causing the case to crack or perhaps some internal components to be jarred loose, then since it is not functioning correctly now, water must have entered somehow.
Either way, due to water ingress or a fall the phone has to be opened to allow the problem to be correctly diagnosed so that it can be repaired.
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%+ to remove all traces of corrosion and water. Do not use "rubbing alcohol" as in some cases this is only 70% and is not as effective. If you do check the label to verify the amount
Here is a link that describes the process.
Repairing iPhone Liquid Damage
You may have to replace the battery as it is very hard to recover a battery from 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.
iPhone 7 Repair
If this process seems too daunting, take your phone to a reputable, professional phone repair service and ask for a quote for a repair. If you decide to do this, do it sooner than later.