Aye, you have to be a member of their club to jump in Vivian. Not like Llyn Padarn across the road has double the depth and roughly the same amount of silt for free.
If my local club weren't taking the Rhib out or I was going to have a nosie on my lonesone/small group I'd be giving GwyneddSAC a shout or a couple pals living in the area with boat access, cheaper and they know the area like the back of their hand above and below water.