Stock view finders on camcorders typically don’t cut the mustard underwater. They are designed to have the eye placed right up against them to be useful. Underwater, the camera is in a housing, and the operator is wearing a diving mask, so the eye ends up too far away to effectively use the camera’s view finder.
Housing manufacturers have come up with several solutions to work around this problem. One is to fit the housing itself with a view finder magnifier. This simply magnifies the camera’s stock view finder making it easier to see underwater and through a diving mask. This is a relatively inexpensive solution. However, you generally need to put your mask right against the housing to see the image.