If you are im See - you are actually in the water, swimming or something.

Being am See - is walking on the shore, laying on its beach or even drinking coffee in a restaurant located am See which has a garden or terrace directly on the bank of the lake...

MichaPau pretty much explained it, but here is my take:

Ich bin am See = I am at the lake [No mentioning of being in it, on it, under it. You are just in the vicinity of the lake.]

Ich bin im See = I am in the lake [You are still at the lake, but you are now probably submerged in the water.]

Ich bin auf dem See = I am on the lake [You are still at the lake, but you are no longer submerged in the water. You are now probably on a little fishing boat, drifting on the water]

So here's the rule of thumb for "Lokale Präpositionen":

an: oceans, lakes, coasts (or something similar) auf = land, islands (or something similar) in = forests, mountain ranges, mountains (or something similar)

Examples using the rules of thumb:

Ich bin am Atlantik - Ich gehe an den Atlantik Ich bin am Meer - Ich gehe ans Meer Ich bin an der Küste - Ich gehe an die Küste (I'm at the beach etc.)

Ich bin auf dem Land - Ich gehe aufs Land Ich bin auf der Insel - Ich gehe auf die Insel (I'm on the island etc.)

Ich bin im Schwarzwald - Ich gehe in den Schwarzwald Ich bin im Gebirge - Ich bin ins Gebirge Ich bin in den Bergen - Ich gehe in die Berge (I'm in the forest etc.)

Hopefully this helped a bit. Remember the rules of thumb and you're golden!