What prep work can I do before transitioning from SEL to SES?
If you already have a pilot license for single-engine land airplanes you've done a huge amount of prep work already - you know all the airspace, navigation, airwork, etc. that you need to know for your seaplane rating.
Brush up on all of that in the seaplane as you can be tested on it and the aerodynamics will be a bit different with floats hanging out there, but you're already well ahead of the game.
You need to learn how to take off, land, and taxi/sail all over again on water (which is arguably the hard part) but the bulk of your existing knowledge will transfer over. 

For studying and prep I imagine the FAA Seaplane Operations Handbook (FAA-H-8083-23) would be the best place to look - it's pretty comprehensive.
Also if you can commandeer a seaplane pilot for some hangar flying (or better a pilot and aircraft for a few real-world touch-and-gos &amp; some practice sailing the plane on water) that would obviously be a great advantage, but coming to your first lesson familiar with the Seaplane Operations Handbook would probably put you well ahead of the curve anyway.