Sorry for the delay in replying - only just noticed I had a PM! Yes, you can swim across to the Mount when the tide covers the causeway - it's only about 500 yards or so. Watch out for the small ferry boats that ply their trade between Marazion and the harbour on the Mount at these times. Tides can get a bit gnarly round the other side, but the harbour side is OK.
Having visited Penzance several times, I have noticed the impressive St Michael's Mount, and have wondered whether it is possible to swim across to it, in the correct conditions? Since you are resident in this area, it seems logical to ask you: I hope you don't mind.
All the best,
Dave_G
