Try the "Blin Hole" between the pier & the lighthouse, most sail straight past, use your sounder to find the "scour" (about 15+ft) posn 51-34-48 by 003-58-20 its always worth a try, especially at low water or on small tides. (whiting dabs, dabs, dabs, & the occasional cod)
For consistent chances of cod if the weather is OK go to 51-33-55 by 003-56-00 (ish) put the S.W.I.G.G. Bouy in line with Mumbles Pier, & be far enough away from the bouy not to be in the shipping channel. the best times are +/- 2hrs either side of low & high water.
Have a GOOD anchor, (& a spare) plenty of rope, & use BIG BAITS, fish a pennel rig with 4/0 or 5/0 hooks plenty of lug & squid, & dont leave your rod unattended!
Look for the local small boat groupings thats as good a tip as you will get