Welcome, Graham,
It's nice to see a new member heading straight for D & A!
"Do the people who set these magazine compet[it]ions have a go themselves, do you think? Just to try out the practicalities?"
(Sorry, Graham, in my role as Deputy Word Nerd Police Officer I had to correct your spelling above.)
But, to answer your question: personally I'd say, "No, I don't imagine they do." I could be wrong, of course.
I think your 'Mariner's Encounter' is a valiant effort at a stinker of a contest, but - for me, at any rate - some lines are just too incongruous in the places you've put them, such as S1L4. It doesn't fit, whereas S2 does. S3 continues the 'story' but then the last stanza makes it go a bit awry.
This is only my humble opinion, FWIW, but I think you have a nice premise here, if you can tweak it into a slightly more coherent form.
Jayne