One can easily ski in September in the Andes (for example, at Portillo) or in New Zealand. As for the Northern Hemisphere, when September would be late summer, the answer varies. Sometimes, snow lasts into September at very high altitude ski areas with glacial fields, such as Mt. Hood or Colorado's Arapahoe Basin. More often, new snow for the new season has arrived in time for some un-lifted skiing on snow fields in the high Rockies; "un-lifted" being that one needs to hike up the snow field. Rarely is there enough skier interest to make it affordable to keep as staff around to run the lifts. Un-lifted skiing can also be practiced sometimes in Scandinavia, Austria, or Switzerland, though lately this has been increasingly rare. Especially in the Alps, the glaciers are shrinking.
You would want to go water skiing when the weather is hot.In really cold weather you can go skiing on the snow.
Most people go water skiing for the fun of it.
You can do heli skiing overseas. You can go to Europe and enjoy heli skiing for a cost. You can check an agency to determine which ski resorts you are interested in and what the cost is for heli skiing.
you can go skiing anywhere in the world that has snow or artificial snow surfaces
That depends on where you are, and whether you're talking about water skiing or snow skiing.
no
Snowboarding can be done on snow. Waterskiing can be done on water.
Whistler Mountain in British Columbia is a popular skiing spot.
Nordic skiing
Heli skiing is done during the winter months because you need snow. Which is why it is called skiing. Exactly when, I do not know. I am not an expert on skiing.
Skiing
Yes.