Here's the rule for JAR Operators:
JAR-OPS 3.843 All helicopters on flights over water - Ditching
(a) An operator shall not operate a helicopter
in Performance Class 1 or 2 on a flight over water
in a hostile environment at a distance from land
corresponding to more than 10 minutes flying
time at normal cruise speed unless that helicopter
is so designed for landing on water or is
certificated in accordance with ditching
provisions.
(b) An operator shall not operate a helicopter
in Performance Class 1 or 2 on a flight over water
in a non-hostile environment at a distance from
land corresponding to more than 10 minutes
flying time at normal cruise speed unless that
helicopter is; so designed for landing on water; or
is certificated in accordance with ditching
provisions; or is fitted with emergency flotation
equipment.
As for your other questions about decks on Bulk Ore carriers etc, the guidelines are in the International Chamber of Shipping helicopter operations guide. Helidecks - in general - are covered by ICAO Annex 14 Vol II.
Last edited by 212man; 28th September 2012 at 03:26.