We only visited to see the tunnels so the beaches were not important, but then having paid for access, it was a little disappointing that only one of the two beaches was accessible as the other had been closed for a wedding ( we were told before we handed over any money).
Spent about 15 minutes there so not the greatest value for money. The beaches are small and comprised of fine black gravel rather than sand but the water looked good for bathers. In fine weather i would imagine the place would be very crowded. At this time of year, there is very little to hold your attention.
The rather short time it takes you to travel through the tunnels is offset by the time it takes to read the many well presented and interesting information panels.