Mata Rocks is nice, but the sea directly offshore is very shallow there, with lots of sea grass. You also need to be very aware of potential boat traffic wherever you swim. Swimming would be easier a bit up the beach at Banyan Bay Hotel, but you could still stay at MR and walk up there - it's only a few minutes' stroll.
June is a very good month weather-wise. Not as blisteringly hot as it gets later on, but well past the windy season and before any significant rains come (in both cases, usually - weather patterns have been rather strange lately).
Welcome in advance!