Surrounded by Water asks the best place to watch fireworks on Mercer Island and my answer is: your back yard! Here's the view from my house on the Fourth evening:
Thankfully Mercer Island remains one of the last bastions of true July 4th Freedom in the area: residents (not visitors!) are allowed to purchase fireworks (in personal quantities only, of course) and discharge them on private property -- not parks or other public spaces, which are specially patrolled this week to prevent violations. Every so often a busy-body will try to shut down our right to peaceably go pyro like this, so if you're a resident please, please don't be stupid or you'll ruin it for the rest of us.