You really need to dive this site about an hour before high water imho. It is an easy entry down stone steps by the harbour wall. Then fin along by the wall on the surface 'till you get to the end and the pier. Then descend under the pier and follow it out as far as you like. On the way back make your ascent under the pier and swim round and back on the surface. Thereby avoiding any overhead boat traffic.
Watch out for fishing lines too.
What I have done in the past is picked a day when high water at trefor was around 2pm so that I could dive Vivien first, get refills for those that need them, then drive down to Trefor in the surface interval to get in the water for around 1 to 1.30. I think that high water this coming Sunday should be handy for that but check tide calc first.