Clear now.

So, a few things that may be happening at existing boat ramps in lakes during the winter:
1) There is a hard, defined slope from inches of water to feet of water; this area may 'warm-up' a bit quicker than other areas of the lake.
2) There's no weed growth over these ramps, thus allowing fish to suspend without interference from weeds and plants.
3) There may be a clarity and/or contrast due to the hard surface of the boat ramp that allows you to see fish more easily.

Excerpt from Robert Crais' "The Monkey's Raincoat:"
"She took another microscopic bite of her sandwich, then pushed it away. Maybe she absorbed nutrients from her surroundings."