Loop in raw water
supply line, going above the boat's water
line? Yes. Vented? = Not on a manual toilet. You will suck air through the loop vent (if it's working) via the toilet suction, and may not get water at all, if vent is real good. If you don't suck air in this configuration your vent is useless anyway.
HOWEVER, if your raw water
intake is pump driven, then a vented loop between pump and toilet is highly recommended, if not crucial by design. Read the manual.
Normally the vented loop goes in the toilet discharge hose and is primarily for those systems with "Y" valves that discharge to sea. Loop will be before the valve, and thus the tank when selected. This keeps the big blue sea from filling up your boat via toilet reversing siphon. If tank is vented (a few oldies are not guys) then vent in loop is not necessary for those systems never discharging directly to sea (or lake).
For non vented "tanks", the loop will have a hose fitted for fume venting to exterior IF system starts to build pressure; I'll spare the details of how it's valve functions (very old system).
Loops in systems without check valves will be practically located on top of the toilet (okay, behind something out of sight, but very close), as contents of hose prior to vent is expected to backflow into the bowl. ONLY those toilets with check valves at the base of the toilet will be free of at least some reverse flow from the discharge hose.
ALL loops should be above the water line, vented or not, and in certain boats with certain toilets (read the directions that came with yours) the loop should be high enough at some point to remain above the water the water line when heeled under way. I've installed and serviced ALL of these for years and years, much to my displeasure (ha ha).
no discharge zones, you need no "vented" loops if tank is vented and y valve is completely disabled or removed (know these laws well), but you may need loops in or out depending on design of toilet and its ability to check itself. However, room for thought... Would you want to trust that valve to remain free of defect when the safety
of your boat is concerned? I'd recommend loops unless instructions specifically say adding such will prevent toilet operation. To date, I personally have seen none that specify this. If I ever run across one that says no loops, I will recommend use of another toilet. Good flushing!