Well...this is a bit difficult. The basic answer is no, there is no difference. The practical answer is that you would only ever say (NO watashi wa...if you're talking about yourself, it's obvious and you don't need to especially point it out) genki desu (polite) and genki (casual). Da is the dictionary form. When you're speaking casual Japanese, you don't say the dictionary form. So, when it's formal (people you aren't close with) you say "Genki desu" and when it's casual (people you know well) you say "Genki!" Never say "genki da" because it's totally redundant. Just say "genki" when you're talking to your friends.

In addition to answers of Matt-san and Ryoko-san, I would like to answer below. da<だ>is an auxiliary verb means conclusion. ※nitearu<にてある>→dearu<である>→da<だ> desu(→des)<です> is an auxiliary verb means polite conclusion. ※nitegozaarimasu<にてござあります>→degozarimasu<でござります>→degozansu<でござんす>→deansu<であんす>→deesu<でえす>→desu<です>（major etymology） "desu" can not be used inside indirect quotation. watashi wa/ga genki da to omotte ita no desu ne.<わたしは／が　げんきだと　おもって　いたのですねHave you thought that I had been fine, haven't you?>　 right watashi wa/ga genki desu to omotte ita no desu ne.<わたしは／が　げんきですと　おもって　いたのですね>　wrong

As Matt said, the difference is between them is formarity. For us, in casual conversation, げんきだ sounds a bit strong and we say げんき or げんきだよ with suffix よ. By the way, this だ is used for NA-adjective when the sentence ends up with it, you can see it on articles, thesis, novels, comics, etc. If you need more details, please wait for the more professional answers. I hope this was hrlpful.

If you look towards the bottom of this grammar guide article, http://www.guidetojapanese.org/learn/grammar/polite it explains that です and だ are different, and it gives some examples too of why they are different. Hopefully this helps!

I would like to answer again. genki<げんき> has two parts of speech. One is a noun and another one is a stem of na-adjective. So the explanation in my first answer fits to noun and does not fit to na-adjective. "genkida" in " watashi wa genki da " is a na-adjective. It's one word. I should have written ＜"genkida" in " watashi wa genki da " is a na-adjective. But the part of da(だ) is the same as da<だ> of an auxiliary verb＞ first. I wrote "indirect quotation" in my first answer. I used it as a wide meanings. I give examples more. ・haha wa watashi wa/ga genki da to tsutaeta<ははは　わたしは／が　げんきだと　つたえたMy mother informed that I was fine.>. right ・haha wa watashi wa/ga genki desu to tsutaeta<ははは　わたしは／が　げんきですと　つたえた>. wrong ・haha wa "Juan wa genki desu" to tsutaeta<ははは 「ホアンは　げんきです」と　つたえたMy mother informed "Juan is fine">. right ※direct quotation ・hahawa watashi ga genki(da)ka dou(da)ka wakaranai to kotaeta <ははは　わたしが　げんき（だ）か　どう（だ）か　わからない　と　こたえた>。My mother answered that she didn't know whether I was fine or not. right ・hahawa watashi ga genkidesuka doudesuka wakaranai to kotaeta <ははは　わたしが　げんきですか　どうですか　わからない　と　こたえた>。 wrong