The traditional hold open latch solution involves a mechanical catch as shown below. These solutions also serve to keep the rub rails from smashing together. But I have always found them difficult and sometimes dangerous to use, especially in rough conditions when the door can get away from you as you reach for the externally mounted catch when standing in the cockpit of the vessel.
Other magnetic catches do exist for marine doors. For example, here is a Whitecap model 6037 that promises to "hold a door open in rough conditions". But this is not for transom doors which are far too heavy (60+ lbs) to be held in place by such small magnets. Besides, even if these magnets were strong enough, you would have to overcome the full force of the magnets in order to break to door free.
The novel solution that we came up with for Third Wave allows the person who is trying to close the transom door to grab onto the big main latch handle and give it a 90 degree turn. This completely disengages the magnets and allows the door to be closed without either pulling hard on the latch or having to fumble with a small latch mechanism in the blind (i.e. without a direct line of sight to your hand when doing it). Often times in rough conditions you will unlatch the door and the door will come free only to slam back shut, perhaps with your hand in the way, if the opening of the latch is not perfectly timed with the pitch and roll of the vessel.
Check out this video on the Third Wave YouTube channel to see how it works. I believe it is an original one of a kind implementation for this application.
