Smell the water cautiously, by doing the thing where you waft the air over it onto your nose without sticking your face over the water.
You waft the lake scent toward you. Although you've smelt a real lake before, it smells fine. Still seems harmless.
(Quote didn't work. This next part is in reply to Enderdash's post.)
Not entirely sure how to start, you begin to gather some twigs and sticks for fuel. Then, you found some dry leaves and grass as tinder and kindling. After about twenty minutes of uselessly scraping twigs together, you give up. If only you had steel, then your flint spear could light a fire.
take your spear and look off into the distance to try and spot a few tribes
You arm yourself with the spear and begin a search of the immediate area. You quickly find a clear and high up area near the lake. It gives you an easy view of the surrounding forest, but you can't see what's under the trees.
Throw a rock away from you into a bush in the opposite direction to spook the deer into coming near you.
You grab a small stone from near you. It's small enough to throw and large enough to make a noise. With a flick of your wrist and a wave of your arm you faceplant into the dirt. Spitting out dust and leaves, you manage to see the deer as they run away. Soon you feel something drip down your face. Great. A bloody nose, you dropped the walnuts, and your covered in itchy leaves.