In regards to hitting a waypoint that you marked...
If you are running only on your unit's internal antenna, that could be part of the issue. A separate heading sensor is best for getting your boat lined up exactly right.
I'm not familiar with a Garmin, but on a Lowrance and Humminbird one of the data overlays you can add is the course/heading/direction (I can't remember exactly what it's called). You can then click your waypoint, and it will tell you which heading to take to hit the waypoint exactly. I find it quite cumbersome, but I have seen guys do it well.
This assumes that your waypoint is in the right spot to begin with. WTM gave good info on that.