Any device that came into contact with water, especially sea water, must be properly cleaned and decontaminated before anything else. Otherwise, you can have all kinds of latent issues down the road. However, with the iPhone 7, Home Button issues are essentially the one thing DIY & third party repair shops cannot address.
I would take a look at this previous question regarding iPhone 7 Home Buttons, particularly @rany 's answer, which is excellent iPhone 7 home button not working after screen replacement
In your case, you could have a logic board issue which may be repairable but if in the end your Home Button is defective, only Apple will be able to solve your problem.