Venice

Should be. I stayed around the Arsenal and found the area less touristy, quiet and comfortable. However, full disclosure that was 12 years ago pre Airbnb.

I didn't come across any places that weren't OK to spend the night.

It sees it is not as easy to get to St. Marks Square or to the airport via ferry, is that.correct?

Not as easy as from where? Arsenal is no more or less difficult to get to the airport than anywhere else in Venice. It is a not too far walk to San Marco, or if you are by the Riva, a short vaporetto ride. But "area west of the Arsenal" still is a big area. Where exactly west of the Arsenal? If you are close to the Riva, transport is easier than from the interior of the island, but that is true throughout Venice.

To stay overnight?

Just one night? Barely worth the bother.

If just staying one night, stay near Piazzale Roma where you can easily to the airport on the bus. Its still Venice.

not as easy to get to St. Marks Square

On the contrary it is very easy: walk south until you reach the harbor, turn right and walk until you see St. Marks Square.

How many times have I been told "it's an easy walk" in Venice . . . and then got lost (it's the easiest thing to do in Venice) and spent way too much time and been way too frustrated . . .

just sayin' . . .

can you narrow down the area of which you speak please? Just saying west of Arsenale is a huge area. Do you mean near the water or in the centre? Which sestiere?