All those things that are out in the wild blue yonder and haven't been washed up on the beach (see beachcombing project). There is obviously a bit of an overlap with beachcombing, but let's say that if it is actually under water at low tide then it's 'mariners' and it can also be beachcombing if you see it from the shore without getting your feet wet. Obviously we are now also including things flying above the wild blue yonder.

The thumbnail image of a dolphin is from an observation by Eve Manning. less ↑