We have a boat collapsible rope and plastic ladder that can be thrown over the side. With that I think we would be in technical compliance for having a re-boarding device. A solid ladder would be better. After that, if you don't have a ladder, there are the better than nothing and may not be in compliance methods. You should have a ladder but if you end up in the water on someones boat without one, turn the outboard motor/s off, then slide yourself up onto the full boat width and very large Al pod deck, like the one we have, which is just barely above sea level, them stand up and step over the transom. Same if it has a near water level swim grid but no ladder. A related method is to turn off the outboard motor/s, them step on the outboard cavitation plate and then push up and over into the low motor well and then into the boat or onto a pod. In my foolish youth we did the outboard cavitation plate method a lot while water skiing - should of had a ladder.
Another method that is better than nothing, if you have no ladder, outboards, pod or swim grid is to have a rope from bow to stern with enough bow in it that it can be pushed down into the water with your hand about a foot and a half to two feet when needed, then step onto the rope to help propel you to step up and over the side of the boat. Not as good as a ladder and dependent on the boat not having to much free board unless you have a 2nd rope over the side with a foot loop on it. You may need to be at least somewhat fit, but better than nothing if you end up in the water on someones boat and they don't have a ladder, no outboard etc. The main point is; get a ladder.