How do side mount ladders work? Storage? Deployment? I have never seen a permanently installed one.
They generally drop into a couple of on-deck hard points, and hang over the side with folding standoffs against the hull. The cheap ones are aluminum with plastic treads, more costly Stainless, good ones have a folding drop down section for extra depth. They are rather long and cumbersome to store, the aluminum ones are weak and can fail at the standoff point with too much side loading.
I've seen versions that clip into points on the hull side below the gunwales too..
I've also seen versions that fold up completely into a rather bulky section of stanchions, effectively making up part of the lifeline structure. Generally they won't go particularly deep.
Boats with high freeboard often use short versions of these to make it easier to step aboard from a low floating dock. On large yachts these can take the form of fold-out platforms c/w swim ladders attached.
For most practical use the mounting points should be port and starboard so if you're rafted or docked you could still have a place to use the ladder. But ultimately it's the stern ladder that will be almost always accessible and instantly deployable.
Here's a link to some side mounted types:
Cabela's: Search Results