Waterproofing the iPhone 7 Plus for use in sea water
The iPhone 7 Plus is splash, water, and dust resistant and has a IP67 rating under IEC standard 60529. Which means up to 30 minutes under water at up to 3 feet deep. 

If you are not submersing your iPhone 7 in more than 3 feet of water for more than 30 minutes you shouldn't need to do any additional waterproofing. However even though the iPhone 7 Plus is has a water resistant rating, water damage is not covered under warranty. You should also note that Apple has specific instructions for drying out a wet phone. See this Apple Support page for all of the details.

Personally, I would invest in a waterproof case to protect a device that costs at least $869, especially if I was planning on exposing it to salt water.

Many "field" tests have been done and the iPhone appears to exceed this rating when new. If you google around a bit you can find videos of iPhone 7 immersion test videos.

Here's a quote from Apple's official iPhone 7 page:


  iPhone 7 and iPhone 7 Plus are splash, water, and dust resistant and were tested under controlled laboratory conditions with a rating of IP67 under IEC standard 60529. Splash, water, and dust resistance are not permanent conditions and resistance might decrease as a result of normal wear. Do not attempt to charge a wet iPhone; refer to the user guide for cleaning and drying instructions. Liquid damage not covered under warranty.