Salt water damaged phones are incredibly difficult to repair. The salt does enormous amounts of damage, even with very little water ingress. So even if the logic board is repairable, you have to replace so many modular components, such as the screen, battery, cameras etc. that it's usually not economically viable.
While you could be lucky and the damage may be very localized, you should focus on recovering the data only. When doing a data recovery effort, we can install modular components temporarily until we can find the fault that is keeping the device from booting. We don't need to fix all of the peripheral functions, it just has to boot and be recognized by iTunes to extract the data.
Look locally for a reputable shop that does micro-soldering repairs. Ideally, you want to find someone local but good workmanship trumps locality. There are many shops worldwide that do this so you shouldn't have any problems finding them.
If you can't find one locally, then consider mail-in repair. There are several reputable micro-solderers patrolling this forum. You can click on my profile to learn more about my services, at the very least, it can serve as a data point.