Boat plumbing and house plumbing have almost nothing in common. Drains on boats don't go into a sewer, they drain overboard...so there's no need for a trap to block sewer gasses. Boat plumbing, except for tank vents, isn't vented either. In fact, a trap would only create a place for water to sit, stagnate and stink...and they don't dry out, either, because the environment on a boat is so humid. And if they freeze, a pipe CAN burst. It's not cold temps that cause freeze damage...when water freezes it expands. Ice needs 110% of the space that liquid water needs.
If anything is needed in some lines connected to a thru-hull, it's a vented loop--just the opposite of a trap, for the very reason that water can't sit and stagnate in a loop.
A bilge isn't a sewer either...although, unless bilges are kept clean they can turn into "swamps" that can make a whole boat SMELL like a sewer. But traps in drains wouldn't solve that problem either...only diligent maintenance does. So if I were you I'd remove the trap and try cleaning your sump a little more often. A weekly dose of Raritan C.P. is all that's needed to keep a sump clean.
I only reported what has worked as a permanent solution to a problem.
a. Freezing won't damage a hose. Yup, I've replaced enough water pipes in refineries to know the ins and outs. As I said, though, a few ounces of PG will remove any concern.
b. If the lines are connected downstream of the trap, the plumbing is the same, only the vent is through the hull instead of a roof vent.
c. Correct, traps do not dry out on a boat. However, like a home, it is required that the trap be rinsed with enough clear water that it is not an odor source, the same as at home (kitchen sinks often stink for this reason).
d. Most boaters do not visit weekly, so that chemical schedule is not practical for them.
e. Yes, there are places on boats where traps and loops are terrible ideas; bilge pumps and scupper drains come to mind. Everything on a boat needs thought-through.
f. A vented loop isn't the opposite of a trap; it's unrelated in this application. You could have both and they would not fight each other. In fact, in a house that (a roof vent) is basically what you have, for a similar reason.