You will love the Serenity with about a 36-39 cm weed fin.
I've used weed fins down to 28 cm (Tangent Reaper) and that really was below the
"good performance" threshold, but I've spent alot of time with 32 cm Tangent Reapers and 32 cm Lessacher Duo's in the Serenity and the board perfroms really well.
In my opinion, it's much better with a 58-62 cm formula type race fin in deep water and just good with a 32-40 cm weed fin in shallow water or weeds.
Hope this helps,