Seaweed extract has been found to have anti-inflammatory properties.
An Australian study has looked at the effect of some of these extracts on knee osteoarthritis
They found very promising results but the results need to be taken with a grain of salt because:
It was a very small study (only 12 subjects)
The study was not blinded and did not have a control
It was funded by the manufacturer.
The results therefore are nothing more than an indication that more research is required. As the authors note, it is time for a double blinded, randomized control study. If the results are as promising in the next round everyone will stand up and notice.
See the study here.