Serenity is lovely however is about 30-40min drive from Patong – the main party beach/area, so you are correct in that you cannot really walk to clubs etc. I woudl say its more suited t a relax holiday.
The beach isnt very nice to swim in and its not advertised this way however they do have a dialy shuttle to take you to a close by beach with is very nice.
March should be fine though… i dont think its the rainy season, might just be hot.
Patong is what you woudl be looking for or Kata or Karon which are close by.
Personally i dont like Patong – there is too much going on.