A 5gal bucket with about 5 or 6 feet of hose connected to the tranny cooler gives you the best of both worlds. Waterhose fills the bucket and the boat pulls from there like it would from the lake. If everything is working correctly it takes about a minute at idle to drain the bucket.
Before that I used the Perko Flush and it works fine, you can't trouble shoot cooling problems though because your forcing water into the motor with enough pressure to temporarily fix any problems on the suction side. I did that for 3 or 4 years and never had an issue with the raw water pump on the Ford motors, GM's use a different pump that may be less tolerant of that. If you have a Vdrive there is pro ally 2 or 3 gals of water in the hoses before the motor so I wouldn't worry with the dry start, but I would put a ball valve on the water hose so I wouldn't have to run at all to turn it on. There is probably 15 to 20 sec. of water in the hoses.