Hi James,
I'm fairly new to spearfishing myself, into my second year now!
As for getting snorkel and fins to check the water for cliff jumping, you're very rarely going to see your hand in front of your face down the gower! I've got a caravan in port eynon so have spent years of summers down there and we have to have weeks of good weather to have any visibility at all!
Haven't heard of anyone spearfishing around mewslade, although I'll be going around there this year for sure weather permitting!
As for cliff jumping, I know some little jems, mostly only accessible by boat mind, but plenty near mewslade as I'm assuming you know where that is! My only advice would be dont jump anything below a 10 meter tide!! Had one of my mates air lifted off Horton beach a few years back for jumping on a small tide. Locals keep hush hush because they don't want people flocking to their favourite spots but if you go on a coasteering day they should show you some safe ones.
As for starting spearfishing, people on here will tell you to get all sorts of kit, just get a snorkel and goggles and next time you're down port eynon have a look on the right of the boat lane towards the rocks, can find crab and see mackerel darting about, and if you feel comfortable buy a speargun, can pick them up new for £50 so not losing too much if you never pick it up again!
Hope this helps, as for fishing off the beach, that's an old man's game and a I'm only 20 so I've got a couple of years yet! Last weekend on night high tide, 6pound bass was caught along with a few smaller ones, Horton side of the beach!
Cheers ash!