My pond that is about 600 gallons, with a bio-filter, pump, mini-stream, and waterfall. About 70% of the surface area of the pond itself is covered with floating plants - water lettuce and parrots feather. Another 15% of the surface area is directly over lush bunches of anacharis which are anchored to the bottom of the pond, but seem to barely reach the pond's surface.

Im in the los angeles area, and even though the nighttime temperatures have recently dipped below freezing at night - they never remain below 32 degrees for any length of time. Ive toyed with the idea of getting a heater for the pond, but Im not willing to spend the big bucks that I'd need to spend to, effectively, purchase a giant aquarium heater - since the floating ice-breaker heaters more than likely wont work in my area for heating the entire pond.

SO MY QUESTION IS... do you think that the plants floating on the surface are prodiving any insulation from the cold air temperature to my fish?

What kinds of fish do you have? What is the water temperature? If you have cold water fish like goldfish and koi, there is no need to heat the water. The floating plants will retain some of the pond's heat on cold nights. Water lettuce is tropical and will wilt and die if it gets frost. Parrot feather and anacharis should grow throughout your LA "winter" season. Since you're in a warm climate, de-icers are basically of no use. They only come on when it's about 35 degrees F or colder in the water. You would only heat the pond if you have tropical fish out there. For entire pond heating, the best systems run water through lines under the pond where the water is heated by a gas heater inside. Aquarium heaters aren't strong enough unless the pond is small (less than 100 gallons). They can also crack.