Well the Swellies
They are a narrow section of the Meni Strights, in total about 100 meters wide and run for about 250m long.
The channel through is much narrower than that maybe 25 metes in some places with rocks at each side and is far from stright
Now all this sounds OK but if you look at your tide tables for Liverpool and then North Wales, at each ends of the Meni Strights and look at the time differences and heights you may see why problems can be found.
Without checking there is about 1-30 to 2-00 hours difference and on tides of upto 10 meters height.
The top and botton of the Meni Strights are quite wide, hundreds of meters maybe 1000's, the tide floods down the strights and when it comes to the Swellies things get interesting.
I have never been there at full flood, hence the question but have done the passage in yachts and was only out of the slack water window by 30 min and had 4/5 Kts of flow aginst us. At full flood i dont know what it runs but its fast.
Anyway hope this helps, this area has a bit of reputation (or so I thought) I know people who go round Anglesy rather than get the times right and go through the Swellies.
If you ever get chance I would think it cound be Er interesting I think is the word. I plan to have a look at some point leaving plenty of margin to bail out if need be, When I do I will let you know how things went