Most consumer GPS is updated at 1Hz (i.e. once a second). Some GPS units may "interpolate" your position based on your course and speed. So it smooths it out like a frame adder on your tv might work (but theres no more accurate data).

It may be possible some units might have a fancy 2 or 5Hz chip, but I don't know how rare they are in boat GPS's.

My GPS updates at 1 second and seems fine, while others seem to have a lag of a second or two before they plot the point which can be extremely frustrating when trying to precisely get on a spot.

So unless these other GPS's have a fancy 5Hz chip, their continuous will probably be the same as your once a second?

 That's good info hlokk. And what about tolerances I've heard they can be out up to ten metres but the more sattelites the better the fix?

not only the more satellites the better but it depends on thier position as well.

i.e 10 satellites with them all reasonably directly overhead may have more error than 5 sprean out across the sky. You'll need at least three to triangulate your position.

Cheers