The term derives from an old, pre-electric, lighting technique: the use of floating candles or oil lamps in a tray of water across the front of the stage. Much later, microphones positioned across the front edge of the stage became known as Float microphones. The use of microphones in this position is now less common than it once was thanks largely to the growth in popularity - and reduction in cost - of radio microphones.
Before rifle mics became popular in this role it was common to use cardioid microphones. One technique involved placing them close to, and pointing down at, the stage surface - sort of using the boundary effect but before the existence of purpose made boundary microphones. AKG C 451's inserted into foam 'mice' were a popular choice.
By the 1980's rifle mics as "float mics", i.e. across the front were the norm for covering ensemble scenes when only a few principles were on radio mics. Some shows where only light amplification was desired were done entirely using this technique. Sometimes the float mics would be supplemented by more microphones overhead and /or firing in from the wings as well.
Later on boundary layer or pressure zone mics became favourites for the float mic position with various types ranging from the Crown PZM through the inexpensive Radio Shack version, the Milab MP30, and probably some others each enjoying their time as favourites and culminating in the Crown PCC 160. But, whatever type of mics are used you can not get the same "in your face" vocal levels that you get from personal radio microphones, unless someone is crouching down or lying right in front of one of the mics of course!
The other accompanying "norm" to the float microphone technique was judicious use of 30 band, 1/3rd octave, graphic equalisers to get rid of all the unwanted high and low frequencies and any frequency response peaks that would otherwise cause major feedback problems. The danger here is that in trying to get rid of everything that wants to feedback you can easily end up with nothing left to amplify, so it is a balancing act - allow for lots of "quiet" time and patience to set this up! The 'old' way with analogue desks was to insert a graphic channel on a group of all the float mics. If there were overheads or other spot mics they would need to be grouped and eq'd separately.
The key to success is mostly in the operating technique. You have to ride the faders constantly because the more mics you have live the less gain before feedback you get, the more noises-off you pick up and the more phasing effects you will get from anything that is being picked up by more than one mic. It's a bit of a lost art, but quite satisfying to do well.
It will also help to enlist the directors help - if the production wants, or needs, to go this way then they need to look at the blocking of scenes with sound in mind. e.g. If you have a small group in a scene then don't put them miles up stage between the axis of two microphones. Get them moved so that they are closer to, and on axis with, a (some) microphones - or put in an extra spot mic somewhere just for that scene.
Colouration & Spill
The other issue to consider is what is off axis from the mics? If you have a loud pit band behind or adjacent to the float mics then they are going to change / colour the sound/balance of the band mix every time you open them up. A mic which has a lovely flat on axis frequency response but horribly colourful side lobes and back lobes might be a hindrance rather than a help.
If you are using rifle mics then something like the Sennheiser K6/ME 66 or MKH 416 or better still MKH 60 for float mics. Save the longer ones e.g. K6/ME67, MKH 816, MKH 70 for overheads etc.
The main advantage of the boundary mics over all the other types is that you don't have the problems of the reflected sound from the stage surface bouncing up into the (rifle or other) mic and interfering with the direct sound because a boundary mic is on the stage surface.