We stayed in Amanpuri this last early April. The rain was definitely off and on and there is a certainly risk you'll lose a whole day (or mulitple), but also plenty of times where it rained hard for an hour or two then subsided. We had rain pretty much the whole afternoon on the day we arrived, and another 4 hours or so of rain one evening. Otherwise, it was spotty and not too disruptive. (Stay was amazing, and we were on the villa side which was absolutely fantastic.) We also visited Amanpulo on this trip and had clouds/rain for about 50% of the time we were there - again, trip was fabulous overall, but if this is a 'big deal' trip where you're going to be disappointed if the weather doesn't break your way, and you have flex on dates, you may want to avoid the rainier seasons in this part of the world.

Worth noting that you probably want to avoid the time around Songkran, as the resort seems to welcome a bunch of large groups then. (We departed the day before Songkran, so wasn't an issue for us, but on our last night you could definitely tell and the common areas were struggling a bit.)