In my mad youthful days, I used to fish Roker (quite successfully) in some massive seas, but one really sticks in my mind.
Myself and my mate(Mick 65) went down an hour before dusk, and climbed over the pier gates at low water. There were waves splashing up as per normal for a force 4 Northerly, but we were used to that.
We got on the roundhead and starting fishing. After about an hour, we notice the troughs of the swell were getting deeper and deeper, the peaks higher and higher, so much so that an hour after low water the top of the swell was about a foot from the top of the roundhead

.
Now we were quite experienced at judging swell size and frequency by then, so we decided on a hasty retreat. We done our usual studying of the swell breaking over the pier and legged it at their lowest point. Fair to say I was bricking it.
I'd love to tell you all that it stopped me and Mick65 doing it again, but it didn't, but I'll never ever forget how menacing that sea looked.