From DE
You can hold a conversation with a body of water. A body of water usually knows about things directly in contact with it, such as boats and fish. Genuine lakes are too complex for this spell to affect; they are not Individuals of Aquam.
(Base 15, +1 Touch, +1 Concentration)
I think Genuine Lake is some form of typo and the author means seas or oceans.
Even a small lake is much bigger than one Individual of Aquam. You'll need Size magnitudes added to pull it off, or just use Part to communicate with a small portion of it.
We have above from DE "Genuine lakes are too complex for this spell to affect; they are not Individuals of Aquam." This says, that just adding size modifiers does not make the spell work on a genuine lake.
Using a spell with T: Part should be possible on a lake, just like in Sense the Feet that Tread the Earth (ArM5 p.154) T: Part eschews the need to define a targeted Individual of Terram. Note that DE has for that spell: "This spell is derived from the Terram tradition from which Guernicus the Founder came, and does not fit well into Hermetic theory."
Voice of the Lake appears similar to me.
Voice of the Lake doesn't have that clause, and therefore, it is within Hermetic theory. And too complex is because you need something like Group and Size modifiers, to be able to affect the various currents and the sheer size of a lake, or (if you have Atlantean Magic) just use Body-of-Water, and some Size modifiers, depending on size of the lake.
An elephant is not more complex than a mouse - but still you need a size modifier to enspell it with Animal. "Too complex for this spell to affect;" is something else.
I think the point of "Genuine lake" is to distinguish a real lake from a slightly oversized pond which locals may refer to as a lake. I live in Kansas but as a child would spend summers in Minnesota, there is a huge difference between what someone in Kansas might refer to as a lake and an actual lake which is miles across and you may not be able to see every side at once.
Please note that this spell is an Intellego spell, therefor "However, the size of the target does make a difference to the level of the spell, with the sole exception of Intellego magic." (Ars CRB. p. 113.)
So problem is not the Size itself - maybe a large lake can be devided into smaller sub-lakes and smaller part-lake spirits one can converse with? 
Just like a big mountain range is divided into several mountain peaks ?
I can imagine there is a giant spirit of the mountain/lake - usually dormant - and parts of that natural formation are governed by lesser spirits. Those can be contacted easier then the dormant greater one.
You need Group. To be able to affect any large body of water, you need to be able to affect all of the individual currents. This is clearly stated for rivers. Actual lakes will run into the same issue of having multiple currents.
On the grimoire project while doing Aquam I did the math for large bodies of water and how much Size it would take to affect them, all the way up to all the oceans of the world. Even a smallish lake can take a surprising amount. Not an issue for Intellego but important for other spells.
As Toronus pointed out, a "genuine" lake is not a large Individual (which could be affected without size modifiers by an Intellego spell), it is a Group. One consequence is that Voice of the Lake does work when cast on a lake or on a sea, it just affects the "local portion" of it (an Individual) rather than the whole (generally a Group). So if you cast it on the sea-water near a beach of the Mediterranean, it will be able to tell you about stuff there (no need for T:Part) but not about stuff on a distant shore many miles away.