it is unlikeli to encounter 8m significant wave height with 7s period. 7s is associated with something like 3.7m, and 8m significant with about 12s (http://www.scribd.com/doc/16545847/I...assenger-Ships
Depends of course on current, wind, fetch, depth and exposure time, but still that is unlikeli.
I have a Parker 6.3 with inboard Diesel since April, using it in the mouth of River Elbe. I have not seen the rough yet, but it makes a pretty sturdy impression. I have no problems in taking it out to the island of Helgoland (35nm from the mainland).
Why not get in contact with Andy from Parker UK:
Andy was very helpful to me.