I dropped mine off at 10:00 am on Saturday and they called at 2:30 pm to let me know it was done and I am sure it wasn't the only boat being worked on. The oil change is easy, I have a pump for the engine oil that sucks it out, replace a filter & fill it with oil you are done. The leg same thing drain it & fill it back up from the bottom using another pump. The only reason I have them do it once a year is for warranty reasons.
I opened the water drain valve on mine and it seemed to drain, a couple of gallons of water came out anyways. I looked at a flushing unit at the dealer but they were none to helpful on how to use it ( I wouldn't be either if I could charge like they do
). What I assume is you attatch a chunk of garden hose to it and stick the hose in a jug of RV antifreeze and idle the engine which will fill the block.
My old boat you just removed 2 plugs and let it drain I never had any trouble doing it myself nor did I use the RV antifreeze in it. I'm just not comfortable taking that risk with a brand new engine.