Is there a way or setting that I can use to take off from water or already in the air with float planes? All the airports are land based. Some of the planes are amphibious (hope I spelled that  right) and have wheels and floats but some have floats only. I have been on Otter and Beaver float planes and they are my favorite. A co- worker of mine owns a T-Craft airplane and last weekend at the airport she saw one (Brand new) that sold for a cool half million without the floats. Looks like I'll never own one.

You can always create a flight at a landbase aiport, go into slew mode, select a floatplane and slew over to some water. pretty simple

If you have a float only plane you can use Andrew's suggestion to get it in water to begin with. After that always save the game after you land and restart where you left off. I do that with almost all my aircraft and flights.

Also, don't restrict yourself to lakes.  There is some good flying and scenery to be found in taking off and landing on rivers.

Watch out for lakes, you can land in them but if it is not long enough you can't get out again.  Be warned!
I believe they tell you this in the seaplane area in the information section.

I have found to that Seaplane base runways are labled east, west, north and south.  This isn't a true science, but might be helpful in finding a seaplane base.