12-15 swells are not uncommon on the WCVI, but I have never seen swells like that on the inside? But, if you put some winds into the equation, it can get rough... very rough on the inside and "seas" are a different story! I have taken "green" water over the bow on the inside, several times! So, can you get 30' seas on the inside… hmmm, I personally haven't seen that, but; Yep... I think you could, but not swells!
Yeh, I have been in 30 foot seas (that is "seas, not "swells"), and it is not pretty! Swells are not the problem. When I lived in Hawaii, 15-20' swells were common. It isnt't the swells you need to watch, but is the seas!
I have also towed a 19' boat in, in 30' seas... and, that was downright ugly, to the point I was ready to "cut" the line! I didn't know boats could do what I saw that boat do, and still stay afloat! I had never seen a boat on a 90-degree angle before! And, it did get my attention to the point, I had everyone put on their life jackets… it was downright scary!
However, I did find the boat would handle and do a LOT more than I will, or want to! [:I]