I use this web site to get real time data for the sea state before leaving home, its really useful.
In general term of the sea is shown at 0.6m and above its reasonably rough for my 4.5m rib, bigger boats may be ok I suppose.
It can save a wasted afternoon drive to the sea only for find its not very safe to go out.
There are other bouys around the coast that also relay real time data.The link is for Brighton / Littlehampton etc
Just make sure you click on 'today' to get the correct days information.
Try it and post if you find it any good.
I find this really useful.............