OSM Page: http://wiki.openstreetmap.org/wiki/Boat_ramp
Note that the suggested tag is a point not a way. We will need to convert these. How?
HB: Import boatramp_cl as highway=service, then for the last node set "leisure=slipway". (LINZ helpfully has all boatramp ways heading out to sea, so the last node is the wet one)

Here is a script for doing that: (run after linz2osm export but before OSM upload)

If the little icon does by some error end up at the wrong end of the ramp it is not the end of the world, as the rendered icon probably overlaps both ends of the ramp anyway, and it has gotten you close enough. :)

Added vehicle=yes to separate it from the linz boatyard slipway (rail+winch) layer.
Uploaded for Pitt Isl. (Chathams) 3 May 2010 (HB)
Uploaded for Mainland 2010-11-26 in changeset 6466510 (HB)