Hi Ivon,
I've just finished my stint as TO for Aberystwyth Uni club, and the amount of time I've spent in Vivian Quarry over the last couple of years gives you an idea what diving in the sea off Aber is like
The bottom's pretty silty and shelves very gradually, so it takes several weeks of perfect weather before the vis gets more than a meter or so. That said, the best snorkelling I've ever done in the UK was around this time last year off Castle Rocks - dozens of fish, crabs, dogfish and HUGE barrel jellyfish.
There are no charter boats running out of Aber, but both the Uni and Town clubs own RIBs - if you're buying the twins off a club member you may be able to tag along on a dive or two?
Your best bet is Pembrokeshire - only 90 minutes drive south and some excellent diving to be had in the summer (Martins Haven or St. Brides for shore dives, or charter boats out of Dale and Broadhaven). Failing that, Anglesey's a possibility to the north. If you're not fussy about salt, Dorothea and Vivian quarries are 90 minutes north in Snowdonia.
HTH, Rick.