Places like Tulamben it is around 28 so should be ok.
Mola site is at Nusa Penida.
Nusa Penida and sites out of Padang Bai can be much colder, down to say 21ish, cold water upwelling from Lombok strait (which is why the Molas are there anyway). Sites nearer Padang Bai (eg crystal bay, Gili sites) are colder than Bali itself from my exp but still would be maybe 24-25 ish.
You can get things that are like rash guards but made of crushed neoprene which I find quite good (I use something called a radiator top which I actually bought at Tulamben in Bali). I wear these under a wetuit though.
In any case I think with no wetsuit you may find it somewhat cold. I reckon 3mm min or maybe get away with 3/2 (3 body 2 legs and arms which seem popular).
Hope this helps.