Just got back from a two night cruise.

You don't need to vent the loop outside the boat. It's just a return bend
with a one way valve that lets air in if a siphon tries to start. It's
identical to the loop you have, or should have, on the top of the water
intake for the head. In fact, you can use one of those. The closer you can
get the loop to the pump, the less back flow water you'll have because the
vent will let the air in and water on one side of the loop will go overboard
and the other side will run back into the bilge.

Roger