1. How can I determine which floats are approved on my airplane?
First look at the Type Certificate Data Sheet (TCDS) for your airplane. That document will specify the approved landing gear for the airplane. Switching between landing gear specified in the TCDS requires a logbook entry, but not a Form 337.
If the floats you want to install are not listed in the TCDS, you need to look for a Supplemental Type Certificate (STC) that permits installation of the floats on your airplane. STC's are proprietary, meaning that you must obtain the STC owner's permission to utilize the STC. Installation of floats in accordance with an STC may require the filing of a form 337.
If neither an STC nor the TCDS provide for the installation of the desired floats, you can pursue a field approval or one time STC, neither of which is likely to be granted, or you can pursue your own STC for the installation, which will require engineering and flight testing.
You may also be able to place the airplane in the experimental or restricted category with uncertified floats. It is generally difficult to return such an aircraft to the normal category and a experimental or restricted category aircraft is more expensive to insure and less valuable when it comes time to sell.
Supplmental Type Certificates and Type Certificate Data Sheets are available to the public on the FAA's web site, www.faa.gov. SPA staff will gladly assist you in locating these documents. Additional information is available in the Float Manufacturers Guide which lists currently manufactured floats and the aircraft on which these floats are approved.
2. How can determine what modifications are approved on my airplane?
See #1 above...
3. Do "riblets" or "speed tape" products improve seaplane performance?
Riblets or speed tape products are textured surfaces that reduce boundary layer drag. They have been used on competitive sailboats, swimsuits and aircraft, among other things, to decrease drag and improve performance. Although it has been asserted that these products work wonders for seaplane performance, scientific evidence to back this claim is not available. Riblets have been shown to reduce drag by a much as 6% and reduce race times by as much as 2% on racing yachts. Presuming these results can be safely correlated to seaplane takeoff performance, properly designed and applied boundary layer textures may reduce a 20 second takeoff run by 2 to 4 tenths of a second. Presuming an average speed of 20 mph during takeoff (remember that a disproportionate amount of time for the takeoff run is at slow plow speeds), and an equal drag reduction effectuate all speeds, 0.4 seconds will yield a 12 foot decrease in takeoff distance.
Regardless of the performance merits (real or imagined) of these products, it is worthwhile to consider the potential for corrosion. Any tape or coating on a float will eventually allow water to seep underneath, where that water will be trapped against the float and could potentially cause corrosion.
And if you are still interested, you may be disappointed to learn that 3M no longer manufactures this product.