Loons on nests should never be approached. The eggs can cool very quickly and need the warmth of the parents to be successful. Motor boats unaware of the nests can come close and startle the Loon into exiting the nest. It's strong back legs adapted for the water can kick an egg into the lake or crack it because of it's hasty retreat. The wakes of boats are also problematic because they can swamp the nest. This image was taken from a highway with a 600 mm plus a 2x teleconverter. The bird when he or she is feeling threatened will lower it's head and stretch it's neck in an effort not to be seen. It was good that it seemed unconcerned. Each bird will have it's own feelings about what distance they may tolerate, so it is best to error on not getting close and causing the nest to fail. While I was aware enough not to distress a nesting bird, I was new to many thing at the time and did not know what I have since learned from reading.