side view:
                                 _
        bridge                __( )_
           |         \|/   __(      )
ladder     |         -O-  (__________)
 |         |         /|\
 | support |
 |   ||   /                            car
 |   ||  /                              |
 |   || /                     traffic   |  truck
 |   ||| boat                  light    |    |
  \  |||   |                      v     v    v
   \ || \  |                                       z 4
   | ||  | | big boat           |--8-8-            z 3
   v vv  v v   v                |           ____   z 2
_____________________________   8      __  /L|  ]  z 1
     II                I ____\__|_____cfb__'o'-o'  z 0 <- ground surface
   H_II_      BBB   ___I/######################### z-1
___HTTTT___B__BBB__/############################## z-2 <- water surface
~~~HTTTT~~~~~~BBB~/############################### z-3
################################################## z-4


            /-surface       _______ pier     i
 _________ L              ___I__I__         -+-   <-- tree
 ~~~~~~~~~  <-water       ~~~I~~I~~        --+--
 ######### <              #########       ___I___
            \-earth                       #######

the z spacing is mostly like this so the player can climb atop cars, trucks, boats and similar things, and not bump his head into a bridge or a traffic light.

Boats consider water surface like cars do it for ground. and while a simple raft might allow player to walk on the level of the water surface on the boat, a bigger boat migh let him climb to some higher levels or go belowdecks.

Ladders are like stairs, they act as a floor, while aloowing movement between Z levels.