Dai - see earlier comments - I'm probably the last person to be giving snorkelling tips, but as the week just past has turned me into a snorkelling guru, here's what I've done..........
Edit - I'm based in San stefanos in the NW corner so that's the beach shown below. Boat hire available for the beach but can be booked in advance.
From the beach - google earth showed some rocks just offshore so I just set off from the beach. Depth 3-7 m where I've been, 7or 8 as yet unidentified species, very interesting for me as a novice. The beach off achavari shows similar formations so you should have some (IMO) good snorkelling from the beach. Visibility excellent once past the breakers.
From a boat - we hired a boat for the day - 85 euros plus fuel (another 30 but I took a detour) and went to diablos, just off the NW corner of corfu. Anchored between the islands and swam all day - crystal clear water, depth anything from 2-10m - same species, just bigger. We did see an octopus the size of a dinner plate, as well as an unidentified 'eel, type thing that legged it as soon as I got close.