Your vented loop is either missing its air valve--the doodad that's supposed to only let air INTO the line to break a siphon, nothing out--or the air valve has failed. The cure: Replace the air valve.

If you'll do a bit better job of rinsing out your head discharge line with clean water and some distlled white vinegar each time before the boat sits, what's squirting will smell a lot better, even when it's waste that is.