Power checks can be accomplished ""ploughing" through the water. You can get enough power on to do a mag check and cycle the prop, without getting up onto the step. these checks must be kept brief though, as the water will be splashing around, and it's really hard on the prop blades. Such a check is dependant upon having enough space ahead of you, in which you can operate without your full attention outside. if in doubt, the power check may have to be overlooked. The mag check can done with some effectivness at idle, and the aircraft is often operated this way (and with carb heat too, if so equipped) when desiring to taxi with the least possible power.
The difference of float flying is obviously the takoff surface. In some cases, it's useable length will be much greater than the runway you might have operated as a wheel plane. This can be very reassuring, as you can get airborne, without having given up the opportunity to land straight ahead for a longer time. This can make one a bit more bold in being a "test pilot" with an aircraft of lesser known condition. I'll leave it to someone else to speculate on the aircraft charactristic which is most often "tested" this way!
Pilot DAR