Waterproofing the iPhone 7 Plus for use in sea water I use an iPhone 7 Plus with an Overboard waterproof bag.
I take a lot of photos and the waterproof bag is very cumbersome.
I've noticed the only holes in the iPhone are the charging port and the speakers. The rest of the iPhone looks sealed. If I put tape over the speakers and charging port, will that be enough waterproof my iPhone 7 Plus? Are there any other "holes" in the iPhone?
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.