If your vented loop is PVC, what you're missing is the air valve that should be threaded into the hole in the nipple at the top of the loop...but because they're a replaceable part that's usually sold separately, it's often missing. The various styles are shown here: Vented Loops & Y Valves - Toilets - Plumbing & Sanitation - Downwind Marine
The air valve is a one-way valve that only allows air INTO a line, nothing out...it needs to periodic cleaning
and occasional replacement. Do NOT put a vent line on it instead of an air valve...it would only be 1/4" line, so small that waste and sea water minerals would clog it in days...turning your vented loop into an UNvented loop that no longer has any ability to break a siphon. Because it's solved
the problem--squirting--it becomes out-of-sight, out-of-mind, never cleaned.
And btw...vented loops need to be at least 6-8" above waterline AT ANY ANGLE OF HEEL...which, on most sailboats, puts it 2-3 FEET above the top of the bowl.
As we have squished our boat deeper into the water with all the added weight...
Surely you don't mean that a li'l ol' vented loop has raised your waterline!