There are varying degrees of quality in underwater lighting, the cheapest being plastic and the most expensive being cast brass.
I've used Kichler and Dabmar products (high end) in the past, as well as Aquascape (middle of the road). I generally try to use LED rather than MR16 due to bulb life. You'll get about 2000hrs out of an MR16 halogen vs. about 20,000hrs out of an LED system. LED creates no heat, whereas halogen radiates heat.
Ponds don't require multiple lights for best effect. One of the best effects (IMO) is a single cast brass up-light with a wide beam that shines straight up, silhouetting the lily pads. The pond appears to "glow" from below. Additionally, if you look at the light at night, you'll see mosquito larvae are attracted to the light in hordes, and the fish dart in and out eating them up.
A simple wide beam, low voltage LED is often enough to light up a waterfall (preferably from under the cascade),
You'll pay more for a single, large cast brass or marine-grade aluminum fixture ($300-$500)than you will for a set of 4-6 plastic egglites ($100-$200), but the effect (and longevity) are worth the bucks. Like anything else, if you're going to do it, buy the best. If you can't afford the best, then wait until you can. There's nothing worse than buying cheap, having it fail, buying cheap again, having it fail and then thinking to yourself...........hmmm.....let's add up two cheapies and compare it to the one I should have bought.
As to transformers, buy the best one you can afford that is rated for more watts than you think you require. In this way, should you decide to add garden/walkway/landscape lighting, you'll be able to run multiple lines from a singly-purchased transformer. The added advantage being that all lights will come on with a single timer, rather than being individually rigged/timed. Digital photocell-controlled timers are superior to the older clock-face timers as they can be set for time-on (ie. 1-3-6, dusk to dawn. In the event of a power failure, clock-faces need to be reset to correct time, whereas photocell-controlled aren't affected.
Check out 1stoplighting.com. They ship free, there are on-line discount coupons available, they provide excellent customer service and offer contractor discounts. They are cheaper than any retail source.
Do what I do......each year I buy myself a gift that I put under the tree that says "To Me, From Me, Love, Me". I'm always pleasantly surprised and always get exactly what I want.
