Woods Hole has begun a project to document cesium contamination levels across the Pacific over time. Samples done to date can be found on their interactive map here:
Various California universities and Lawrence Berkeley National Lab are starting a program to monitor sea kelp for radioactive contamination. This new program may help identify changes in the Pacific ocean off the US west coast.
Various agencies have done plume modeling estimates. These take weather conditions and releases and estimate where the radioactive releases went or will go.
US State Dept Geographer Fukushima Sea Plume March 2012.
NOAA has a visual plume model of releases toward the Pacific and North America
IRSN plume maps: http://frmr-jp.blogspot.com/2012/09/20113.html
USGS Fukushima fallout deposition over the US (NADP) http://nadp.sws.uiuc.edu/fukushima/
Adrift.org sea plume of Fukushima http://adrift.org.au/fukushima
* they used the program they also use to model sea trash so disregard the reference to “plastics”