Kanai a Nalu is one of my favorite places to stay on Maui; I've stayed there 5 times.
One of my big fears each time was that the dreaded cane harvest would be taking place.
Alas, one May a couple of years ago my luck ran out. First, they no longer burn the cane in the nearby field(s). They cut it with machines and then turn the soil. The problem is dust...not ashes. I was amazed at how quickly and thoroughly the marvelous caretakers at KaN cleaned everything up! The dust only impacted one day of our stay. We kept the lanai door closed, which wasn't a problem because the units have A/C. The lawn and pool area was protected by the building and was clean and usable at all times. If there is ash it is likely from VOG carried over from the Big Island. Rarely a problem in Maalaea unless the winds die off completely.
We enjoy swimming in the ocean right in front ot the complex where wave action is usually light. There are a few rocks, but most of the time the water is clear enough to see and avoid them. We have been there, also, when the waves were stronger and local surfers turned up to put on a good show. There are prettier tropical beaches a short drive away (my favorites are in So, Kihei).
We lay out on the huge lawn area so there is no stinging sand if the wind kicks up. The heated pool is a lovely alternative to the ocean. The view from our lawn chairs are unsurpassed...about 3 ft from the ocean. We enjoy watching the boats come and go from Maalaea Harbor. Mornings are calm; wind picks up mid day. You might welcome strong breezes in September, which is a hot and humid month.
One negative is that the surrounding area (and most of So. Maui) is dry...not green and tropical except on the manicured grounds of condos and hotels. Just look at the ocean instead!