Turn off the power first.
I use a pool vac hose to siphon. It moves a lot of water quickly, so you need to have a place to put that water.
Speaking of which, if your chlorine level is down, you can drain onto your yard area. If the chlorine level is really high, drain onto your neighbor's yard....
Or into the street.
If this is a simple water change, you don't need to get 100% of the water out. But if you were having problems, its not a bad idea to drain the tub as completely as you can: shop vac the seat or other areas which may hold a bit of water, including the jet system, filter areas and open built-in drains and let them empty.