Block the Boat is a protest of the occupation of Gaza. Activists are trying to block an Israeli container ship from unloading first in Tacoma (on 8/22) and then in Seattle (on 8/24).
All the details can be found at http://blocktheboatnw.wordpress.com/
The port of Seattle was shut down by activists in 2011. That protest was a part of coordinated Occupy protests.
There are a number of block the boat protests happening. See this one in Oakland.